feat: Add Angular, HTML, and SCSS / Sass language servers (#1439)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Pavlo Basanets <Pavlo_Basanets@epam.com>

test/solidlsp/html_ls/test_html_diagnostics.py

@@ -0,0 +1,27 @@
+"""
+Diagnostics smoke test for the HTML language server.
+
+``vscode-html-language-server`` does not validate HTML structure itself — upstream
+``vscode-html-languageservice`` only forwards validation to embedded sub-services
+(CSS/JS in ``<style>`` / ``<script>`` blocks), and those aren't wired up in
+standalone mode. So a malformed-HTML fixture would yield zero diagnostics regardless.
+
+Asserting "at least one diagnostic" would be a false positive waiting to happen.
+Instead, this test verifies the diagnostics endpoint is reachable end-to-end and
+the framework's pull→publish fallback handles the empty response correctly — i.e.
+it returns a list, not an error. That alone catches LS-crash regressions on the
+diagnostic request, which is the practical purpose for this LS.
+"""
+
+import pytest
+
+from solidlsp import SolidLanguageServer
+from solidlsp.ls_config import Language
+
+
+@pytest.mark.html
+class TestHtmlDiagnostics:
+    @pytest.mark.parametrize("language_server", [Language.HTML], indirect=True)
+    def test_diagnostics_endpoint_returns_list(self, language_server: SolidLanguageServer) -> None:
+        diagnostics = language_server.request_text_document_diagnostics("diagnostics_sample.html", min_severity=1)
+        assert isinstance(diagnostics, list), diagnostics