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_console_authz_audit.py

@@ -23,7 +23,7 @@ import sys
 import tempfile
 import unittest
 
-from starlette.testclient import TestClient
+from webui_testclient import TestClient
 
 sys.path.insert(0, str(pathlib.Path(__file__).resolve().parents[1]))