Use official .NET install scripts instead of manual downloads

Replace manual .NET runtime downloads with Microsoft's official dotnet-install
scripts for better maintainability and automatic platform detection.

Changes:
- Remove 6 DotNetRuntime RuntimeDependency entries (42 lines removed)
- Replace _ensure_dotnet_runtime_from_config with _install_dotnet_with_script
- Use bash script (dotnet-install.sh) for Linux/macOS
- Use PowerShell script (dotnet-install.ps1) for Windows
- Update version check from .NET 9 to .NET 10+ (supports 10.x, 11.x, etc.)
- Add comprehensive tests for install script behavior

Benefits:
- Simpler codebase (~80 lines of manual download code → ~60 lines of script invocation)
- Automatic platform/architecture detection by Microsoft scripts
- Built-in checksum verification and dependency handling
- Official Microsoft-maintained approach
- Easier version updates (just change version string)
- Better compatibility with .NET 10 (required by roslyn-language-server packages)

The language server packages from NuGet.org remain unchanged - this only
affects how the .NET runtime is installed to execute the language server.

test/solidlsp/csharp/test_csharp_dotnet_install.py

@@ -0,0 +1,145 @@
+"""Tests for C# language server .NET runtime installation using official install scripts."""
+
+import tempfile
+from pathlib import Path
+from unittest.mock import Mock, patch
+
+import pytest
+
+from solidlsp.language_servers.csharp_language_server import CSharpLanguageServer
+from solidlsp.settings import SolidLSPSettings
+
+
+@pytest.mark.csharp
+class TestDotNetInstallScript:
+    """Test .NET runtime installation using Microsoft's official install scripts."""
+
+    @patch("solidlsp.language_servers.csharp_language_server.CSharpLanguageServer.DependencyProvider._ensure_language_server")
+    @patch("solidlsp.language_servers.csharp_language_server.platform.system")
+    @patch("solidlsp.language_servers.csharp_language_server.subprocess.run")
+    @patch("solidlsp.language_servers.csharp_language_server.urllib.request.urlretrieve")
+    @patch("solidlsp.language_servers.csharp_language_server.shutil.which")
+    def test_install_dotnet_uses_bash_script_on_linux(self, mock_which, mock_urlretrieve, mock_subprocess, mock_platform, mock_ensure_ls):
+        """Test that Linux uses bash install script."""
+        mock_platform.return_value = "Linux"
+        mock_which.return_value = None  # No system dotnet
+        mock_ensure_ls.return_value = "/fake/server.dll"
+
+        with tempfile.TemporaryDirectory() as temp_dir:
+
+            def mock_retrieve(url, path):
+                # Create the file that urlretrieve would create
+                Path(path).touch()
+                # Also create the dotnet executable after script download
+                if "dotnet-install.sh" in str(path):
+                    dotnet_dir = Path(temp_dir) / "dotnet-runtime-10.0"
+                    dotnet_dir.mkdir(parents=True, exist_ok=True)
+                    dotnet_exe = dotnet_dir / "dotnet"
+                    dotnet_exe.touch()
+                    dotnet_exe.chmod(0o755)
+
+            mock_urlretrieve.side_effect = mock_retrieve
+            mock_subprocess.return_value = Mock(returncode=0, stdout="", stderr="")
+
+            mock_settings = SolidLSPSettings()
+            custom_settings = SolidLSPSettings.CustomLSSettings({})
+
+            _ = CSharpLanguageServer.DependencyProvider(
+                custom_settings=custom_settings,
+                ls_resources_dir=temp_dir,
+                solidlsp_settings=mock_settings,
+                repository_root_path="/fake/repo",
+            )
+
+            # Verify bash script was downloaded
+            assert mock_urlretrieve.called
+            script_url = mock_urlretrieve.call_args[0][0]
+            assert script_url == "https://dot.net/v1/dotnet-install.sh"
+
+    @patch("solidlsp.language_servers.csharp_language_server.CSharpLanguageServer.DependencyProvider._ensure_language_server")
+    @patch("solidlsp.language_servers.csharp_language_server.platform.system")
+    @patch("solidlsp.language_servers.csharp_language_server.subprocess.run")
+    @patch("solidlsp.language_servers.csharp_language_server.urllib.request.urlretrieve")
+    @patch("solidlsp.language_servers.csharp_language_server.shutil.which")
+    def test_install_dotnet_uses_powershell_script_on_windows(
+        self, mock_which, mock_urlretrieve, mock_subprocess, mock_platform, mock_ensure_ls
+    ):
+        """Test that Windows uses PowerShell install script."""
+        mock_platform.return_value = "Windows"
+        mock_which.return_value = None  # No system dotnet
+        mock_ensure_ls.return_value = "/fake/server.dll"
+
+        with tempfile.TemporaryDirectory() as temp_dir:
+
+            def mock_retrieve(url, path):
+                # Create the file that urlretrieve would create
+                Path(path).touch()
+                # Also create the dotnet executable after script download
+                if "dotnet-install.ps1" in str(path):
+                    dotnet_dir = Path(temp_dir) / "dotnet-runtime-10.0"
+                    dotnet_dir.mkdir(parents=True, exist_ok=True)
+                    dotnet_exe = dotnet_dir / "dotnet.exe"
+                    dotnet_exe.touch()
+
+            mock_urlretrieve.side_effect = mock_retrieve
+            mock_subprocess.return_value = Mock(returncode=0, stdout="", stderr="")
+
+            mock_settings = SolidLSPSettings()
+            custom_settings = SolidLSPSettings.CustomLSSettings({})
+
+            _ = CSharpLanguageServer.DependencyProvider(
+                custom_settings=custom_settings,
+                ls_resources_dir=temp_dir,
+                solidlsp_settings=mock_settings,
+                repository_root_path="/fake/repo",
+            )
+
+            # Verify PowerShell script was downloaded
+            assert mock_urlretrieve.called
+            script_url = mock_urlretrieve.call_args[0][0]
+            assert script_url == "https://dot.net/v1/dotnet-install.ps1"
+
+    def test_uses_system_dotnet_10_if_available(self):
+        """Test that system .NET 10 is used if available instead of downloading."""
+        from solidlsp.ls_utils import PlatformId
+
+        with patch("solidlsp.language_servers.csharp_language_server.shutil.which") as mock_which:
+            with patch("solidlsp.language_servers.csharp_language_server.subprocess.run") as mock_subprocess:
+                with patch("solidlsp.language_servers.common.PlatformUtils.get_platform_id") as mock_platform_id:
+                    with patch(
+                        "solidlsp.language_servers.csharp_language_server.CSharpLanguageServer.DependencyProvider._ensure_language_server"
+                    ) as mock_ensure_ls:
+                        mock_which.return_value = "/usr/bin/dotnet"
+                        mock_ensure_ls.return_value = "/fake/server.dll"
+                        mock_platform_id.return_value = PlatformId.LINUX_x64
+
+                        # Mock dotnet --list-runtimes output showing .NET 10
+                        mock_result = Mock()
+                        mock_result.stdout = "Microsoft.NETCore.App 10.0.1 [/usr/share/dotnet/shared/Microsoft.NETCore.App]"
+                        mock_result.returncode = 0
+                        mock_subprocess.return_value = mock_result
+
+                        with tempfile.TemporaryDirectory() as temp_dir:
+                            mock_settings = SolidLSPSettings()
+                            custom_settings = SolidLSPSettings.CustomLSSettings({})
+
+                            dependency_provider = CSharpLanguageServer.DependencyProvider(
+                                custom_settings=custom_settings,
+                                ls_resources_dir=temp_dir,
+                                solidlsp_settings=mock_settings,
+                                repository_root_path="/fake/repo",
+                            )
+
+                            # Should have used system dotnet
+                            assert dependency_provider._dotnet_path == "/usr/bin/dotnet"
+
+    def test_runtime_dependencies_no_longer_include_dotnet_downloads(self):
+        """Test that _RUNTIME_DEPENDENCIES no longer includes manual .NET download entries."""
+        from solidlsp.language_servers.csharp_language_server import _RUNTIME_DEPENDENCIES
+
+        # Check that DotNetRuntime entries don't exist (we use install script instead)
+        dotnet_deps = [dep for dep in _RUNTIME_DEPENDENCIES if dep.id == "DotNetRuntime"]
+
+        # After our changes, there should be no DotNetRuntime entries
+        # (we'll use the install script instead)
+        assert len(dotnet_deps) == 0, "DotNetRuntime dependencies should be removed in favor of install script"