test(mcp): migrate _kg monkeypatches to _get_kg (#1136)

Direct module-attribute patching of _kg is obsolete after the lazy
cache refactor. Switch test helpers to patch _get_kg instead so the
fixture KG replaces the factory rather than a now-missing singleton.

- tests/test_mcp_server.py: _patch_mcp_server helper
- tests/benchmarks/test_mcp_bench.py: _patch_mcp_config helper
- tests/benchmarks/test_memory_profile.py: inline patch in test_tool_status_repeated_calls

tests/benchmarks/test_memory_profile.py

@@ -84,8 +84,9 @@ class TestToolStatusMemoryProfile:
 
         cfg = MempalaceConfig(config_dir=str(tmp_path / "cfg"))
         monkeypatch.setattr(cfg, "_file_config", {"palace_path": palace_path})
+        kg = KnowledgeGraph(db_path=str(tmp_path / "kg.sqlite3"))
         monkeypatch.setattr(mcp_mod, "_config", cfg)
-        monkeypatch.setattr(mcp_mod, "_kg", KnowledgeGraph(db_path=str(tmp_path / "kg.sqlite3")))
+        monkeypatch.setattr(mcp_mod, "_get_kg", lambda: kg)
 
         from mempalace.mcp_server import tool_status