Switch C# language server from Azure NuGet feed to official NuGet.org

Replace Microsoft.CodeAnalysis.LanguageServer packages from Azure DevOps
feed with official roslyn-language-server packages from NuGet.org.

Changes:
- Update package names to roslyn-language-server.* (from Microsoft.CodeAnalysis.LanguageServer.*)
- Update version from 5.0.0-1.25329.6 to 5.5.0-2.26078.4
- Add direct URLs pointing to NuGet.org for each platform variant
- Update extract paths from content/LanguageServer/<platform> to tools/net10.0/<platform>
- Simplify download logic by removing Azure feed service index lookup
- Replace _download_nuget_package_direct with simpler _download_nuget_package
- Add tests to verify NuGet.org download behavior

Benefits:
- More reliable package source (official NuGet.org vs Azure DevOps feed)
- Newer version with latest Roslyn improvements
- Simpler codebase (removed ~60 lines of Azure-specific logic)
- Better error messages with explicit URLs

test/solidlsp/csharp/test_csharp_nuget_download.py

@@ -0,0 +1,118 @@
+"""Tests for C# language server NuGet package download from NuGet.org."""
+
+import tempfile
+from unittest.mock import patch
+
+import pytest
+
+from solidlsp.language_servers.common import RuntimeDependency
+from solidlsp.language_servers.csharp_language_server import CSharpLanguageServer
+from solidlsp.settings import SolidLSPSettings
+
+
+@pytest.mark.csharp
+class TestNuGetOrgDownload:
+    """Test downloading Roslyn language server packages from NuGet.org."""
+
+    def test_download_nuget_package_uses_direct_url(self):
+        """Test that _download_nuget_package uses the URL from RuntimeDependency directly."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            # Create a RuntimeDependency with a NuGet.org URL
+            test_dependency = RuntimeDependency(
+                id="TestPackage",
+                description="Test package from NuGet.org",
+                package_name="roslyn-language-server.linux-x64",
+                package_version="5.5.0-2.26078.4",
+                url="https://www.nuget.org/api/v2/package/roslyn-language-server.linux-x64/5.5.0-2.26078.4",
+                platform_id="linux-x64",
+                archive_type="nupkg",
+                binary_name="Microsoft.CodeAnalysis.LanguageServer.dll",
+                extract_path="content/LanguageServer/linux-x64",
+            )
+
+            # Mock the dependency provider
+            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",
+            )
+
+            # Mock urllib.request.urlretrieve to capture the URL being used
+            with patch("solidlsp.language_servers.csharp_language_server.urllib.request.urlretrieve") as mock_retrieve:
+                with patch("solidlsp.language_servers.csharp_language_server.SafeZipExtractor"):
+                    try:
+                        dependency_provider._download_nuget_package(test_dependency)
+                    except Exception:
+                        # Expected to fail since we're mocking, but we want to check the URL
+                        pass
+
+                    # Verify that urlretrieve was called with the NuGet.org URL
+                    assert mock_retrieve.called, "urlretrieve should be called"
+                    called_url = mock_retrieve.call_args[0][0]
+                    assert called_url == test_dependency.url, f"Should use URL from RuntimeDependency: {test_dependency.url}"
+                    assert "nuget.org" in called_url, "Should use NuGet.org URL"
+                    assert "azure" not in called_url.lower(), "Should not use Azure feed"
+
+    def test_runtime_dependencies_use_nuget_org_urls(self):
+        """Test that _RUNTIME_DEPENDENCIES are configured with NuGet.org URLs."""
+        from solidlsp.language_servers.csharp_language_server import _RUNTIME_DEPENDENCIES
+
+        # Check language server dependencies
+        lang_server_deps = [dep for dep in _RUNTIME_DEPENDENCIES if dep.id == "CSharpLanguageServer"]
+
+        assert len(lang_server_deps) == 6, "Should have 6 language server platform variants"
+
+        for dep in lang_server_deps:
+            # Verify package name uses roslyn-language-server
+            assert dep.package_name is not None, f"Package name should be set for {dep.platform_id}"
+            assert dep.package_name.startswith(
+                "roslyn-language-server."
+            ), f"Package name should start with 'roslyn-language-server.' but got: {dep.package_name}"
+
+            # Verify version is the newer NuGet.org version
+            assert dep.package_version == "5.5.0-2.26078.4", f"Should use NuGet.org version 5.5.0-2.26078.4, got: {dep.package_version}"
+
+            # Verify URL points to NuGet.org
+            assert dep.url is not None, f"URL should be set for {dep.platform_id}"
+            assert "nuget.org" in dep.url, f"URL should point to nuget.org, got: {dep.url}"
+            assert "azure" not in dep.url.lower(), f"URL should not point to Azure feed, got: {dep.url}"
+
+    def test_download_method_does_not_call_azure_feed(self):
+        """Test that the new download method does not attempt to access Azure feed."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            test_dependency = RuntimeDependency(
+                id="TestPackage",
+                description="Test package",
+                package_name="roslyn-language-server.linux-x64",
+                package_version="5.5.0-2.26078.4",
+                url="https://www.nuget.org/api/v2/package/roslyn-language-server.linux-x64/5.5.0-2.26078.4",
+                platform_id="linux-x64",
+                archive_type="nupkg",
+                binary_name="test.dll",
+            )
+
+            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",
+            )
+
+            # Mock urllib.request.urlopen to track if Azure feed is accessed
+            with patch("solidlsp.language_servers.csharp_language_server.urllib.request.urlopen") as mock_urlopen:
+                with patch("solidlsp.language_servers.csharp_language_server.urllib.request.urlretrieve"):
+                    with patch("solidlsp.language_servers.csharp_language_server.SafeZipExtractor"):
+                        try:
+                            dependency_provider._download_nuget_package(test_dependency)
+                        except Exception:
+                            pass
+
+                        # Verify that urlopen was NOT called (no service index lookup)
+                        assert not mock_urlopen.called, "Should not call urlopen for Azure service index lookup"