fix(webui): migrate Starlette TestClient to httpx2 (#682)

Starlette 1.3.x prefers httpx2 for starlette.testclient.TestClient; plain
httpx still works but emits StarletteDeprecationWarning.

- Pin httpx2==2.9.1 (keep httpx for MCP/runtime)
- Centralize Web UI TestClient import via tests/webui_testclient.py
- Point all test_webui_* modules at the helper
- Add regression tests that the deprecation warning is gone

Closes #682

tests/test_webui_system_health_dashboard.py

@@ -11,7 +11,7 @@ from pathlib import Path
 
 sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
 
-from starlette.testclient import TestClient
+from webui_testclient import TestClient
 
 from webui.app import create_app
 from webui.deployment_boundary import scan_text_for_client_secrets