test(themes): use one import form for theme_guard

CodeQL's py/import-and-import-from failed the gate: the orphaned-task
regression test reached the module as `import
ha_mcp.dashboard_screenshot.theme_guard as guard_module` while the file
already imports names from it with `from ... import`. Switched to the form
the file's other module-level monkeypatch test already uses,
`from ha_mcp.dashboard_screenshot import theme_guard as guard_module`.

Everything else on the previous head was green, including the first real
run of the deterministic dev-mode identity test, the corrected themes
assertion, and the raised performance baselines.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Nm7tyA1nfxNCWFXaR3AxV

Author: kingpanther13

tests/src/unit/test_dashboard_screenshot_theme_guard.py

@@ -545,7 +545,7 @@ async def test_blocked_disconnect_is_cancelled_not_orphaned(
         self, monkeypatch: Any
     ) -> None:
         """A disconnect that never completes must not outlive the cleanup."""
-        import ha_mcp.dashboard_screenshot.theme_guard as guard_module
+        from ha_mcp.dashboard_screenshot import theme_guard as guard_module
 
         entered = asyncio.Event()
         tasks: list[Any] = []