Fix clangd cache invalidation and capability checks (#1359)

Summary
relax clangd initialize capability checks to tolerate valid response shape differences
add a clangd-specific document-symbol cache fingerprint so stale C++ symbol caches are invalidated when backend/config state changes
add a regression test covering C++ document-symbol cache invalidation
Problem
For C++ projects, Serena's symbol tools could keep returning malformed cached document symbols even after clangd / compile commands / backend state had been repaired. The cache key was effectively stable as long as file contents did not change, so previously bad symbol trees could survive later LS recovery.

In addition, clangd capability checks were too strict and assumed a single exact initialize-response shape.

What this changes
accepts both integer and object-style textDocumentSync responses
validates completionProvider more conservatively by checking required trigger characters instead of exact equality
fingerprints clangd document-symbol caches using:
clangd_version
ls_path
compile_commands_dir
compile_commands.json content hash
adds a C++ regression test similar to the existing Go cache invalidation coverage

test/solidlsp/cpp/test_cpp_basic.py

@@ -9,12 +9,14 @@ server is not available.
 import os
 import pathlib
 import shutil
+from pathlib import Path
 
 import pytest
 
 from solidlsp import SolidLanguageServer
 from solidlsp.ls_config import Language
 from solidlsp.ls_utils import SymbolUtils
+from test.conftest import get_repo_path, start_ls_context
 from test.solidlsp.conftest import format_symbol_for_assert, has_malformed_name, request_all_symbols
 
 
@@ -156,3 +158,72 @@ int use_add() {
                 f"Found malformed symbols: {[format_symbol_for_assert(sym) for sym in malformed_symbols]}",
                 pytrace=False,
             )
+
+
+@pytest.mark.cpp
+class TestCppDocumentSymbolCache:
+    def _copy_cpp_fixture(self, tmp_path: Path) -> Path:
+        fixture_path = get_repo_path(Language.CPP)
+        target_path = tmp_path / "test_repo"
+        shutil.copytree(fixture_path, target_path)
+        return target_path
+
+    def test_cache_invalidates_when_clangd_context_changes(self, tmp_path: Path) -> None:
+        repo_path = self._copy_cpp_fixture(tmp_path)
+        ls_settings_alt = {
+            Language.CPP: {
+                "compile_commands_dir": ".serena-alt",
+            }
+        }
+
+        main_cpp = os.path.join("a.cpp")
+
+        def _assert_caches_loaded_and_clean(ls: SolidLanguageServer) -> None:
+            assert ls._raw_document_symbols_cache, "Expected raw document-symbol cache to load from disk"
+            assert ls._document_symbols_cache, "Expected document-symbol cache to load from disk"
+            assert not ls._raw_document_symbols_cache_is_modified
+            assert not ls._document_symbols_cache_is_modified
+
+        def _assert_caches_empty(ls: SolidLanguageServer) -> None:
+            assert ls._raw_document_symbols_cache == {}
+            assert ls._document_symbols_cache == {}
+
+        def _assert_caches_modified(ls: SolidLanguageServer) -> None:
+            assert ls._raw_document_symbols_cache_is_modified
+            assert ls._document_symbols_cache_is_modified
+
+        with start_ls_context(Language.CPP, repo_path=str(repo_path), solidlsp_dir=tmp_path) as ls_default:
+            _ = ls_default.request_document_symbols(main_cpp)
+
+            default_raw_cache_version = ls_default._raw_document_symbols_cache_version()
+            default_doc_cache_version = ls_default._document_symbols_cache_version()
+
+            ls_default.save_cache()
+            cache_dir = ls_default.cache_dir
+            cache_files = [p for p in cache_dir.rglob("*") if p.is_file()]
+            assert cache_files, f"Expected SolidLSP to create cache artifacts under {cache_dir}"
+
+        with start_ls_context(Language.CPP, repo_path=str(repo_path), solidlsp_dir=tmp_path) as ls_default_again:
+            assert ls_default_again.cache_dir == cache_dir
+            _assert_caches_loaded_and_clean(ls_default_again)
+            _ = ls_default_again.request_document_symbols(main_cpp)
+            assert not ls_default_again._raw_document_symbols_cache_is_modified
+            assert not ls_default_again._document_symbols_cache_is_modified
+
+        with start_ls_context(
+            Language.CPP,
+            repo_path=str(repo_path),
+            ls_specific_settings=ls_settings_alt,
+            solidlsp_dir=tmp_path,
+        ) as ls_alt:
+            assert ls_alt.cache_dir == cache_dir
+            alt_raw_cache_version = ls_alt._raw_document_symbols_cache_version()
+            alt_doc_cache_version = ls_alt._document_symbols_cache_version()
+
+            assert alt_raw_cache_version != default_raw_cache_version
+            assert alt_doc_cache_version != default_doc_cache_version
+
+            _assert_caches_empty(ls_alt)
+
+            _ = ls_alt.request_document_symbols(main_cpp)
+            _assert_caches_modified(ls_alt)