fix: add stub add_file_imports/resolve_import to CSharpAnalyzer

CSharpAnalyzer was missing implementations for the new abstract methods
added by this PR, causing TypeError on import.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gkorland

api/analyzers/csharp/analyzer.py

@@ -136,3 +136,11 @@ def resolve_symbol(self, files: dict[Path, File], lsp: SyncLanguageServer, file_
             return self.resolve_method(files, lsp, file_path, path, symbol)
         else:
             raise ValueError(f"Unknown key {key}")
+
+    def add_file_imports(self, file: File) -> None:
+        # C# import tracking not yet implemented
+        pass
+
+    def resolve_import(self, files: dict[Path, File], lsp: SyncLanguageServer, file_path: Path, path: Path, import_node) -> list[Entity]:
+        # C# import resolution not yet implemented
+        return []