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/webui_testclient.py

@@ -0,0 +1,20 @@
+"""Canonical Web UI ``TestClient`` import for the Gitea-Tools suite (#682).
+
+Starlette 1.3+ builds ``starlette.testclient.TestClient`` on ``httpx2``.
+Plain ``httpx`` still works but emits ``StarletteDeprecationWarning``.
+
+All Web UI tests import ``TestClient`` from this module so:
+
+* dependency intent is single-sourced (``httpx2`` in ``requirements.txt``);
+* a future client-construction helper has one place to land;
+* import-site audits do not need to re-scan every ``test_webui_*.py``.
+
+Runtime MCP / FastMCP code continues to use ``httpx``; this module is
+test-only and does not change production clients.
+"""
+
+from __future__ import annotations
+
+from starlette.testclient import TestClient
+
+__all__ = ["TestClient"]