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_analytics.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 import os
 import tempfile
 import unittest
-from starlette.testclient import TestClient
+from webui_testclient import TestClient
 
 import control_plane_db
 from webui.analytics_loader import (