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_issue_682_starlette_httpx2.py

@@ -0,0 +1,66 @@
+"""Starlette TestClient uses httpx2 without deprecation warning (#682)."""
+
+from __future__ import annotations
+
+import importlib
+import sys
+import unittest
+import warnings
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+sys.path.insert(0, str(Path(__file__).resolve().parent))
+
+
+class TestHttpx2Installed(unittest.TestCase):
+    def test_httpx2_importable(self) -> None:
+        httpx2 = importlib.import_module("httpx2")
+        self.assertTrue(hasattr(httpx2, "Client") or hasattr(httpx2, "AsyncClient"))
+
+
+class TestNoStarletteTestClientDeprecation(unittest.TestCase):
+    def test_starlette_testclient_import_does_not_warn(self) -> None:
+        # Force a fresh import path so a previous httpx-fallback warning cannot
+        # hide a regression after httpx2 is present.
+        for name in list(sys.modules):
+            if name == "starlette.testclient" or name.startswith("starlette.testclient."):
+                del sys.modules[name]
+
+        with warnings.catch_warnings(record=True) as caught:
+            warnings.simplefilter("always")
+            import starlette.testclient as testclient  # noqa: F401
+
+        dep = [
+            w
+            for w in caught
+            if "httpx" in str(w.message).lower()
+            and "deprecated" in str(w.message).lower()
+        ]
+        self.assertEqual(
+            dep,
+            [],
+            f"expected no Starlette httpx deprecation warning; got: "
+            f"{[str(w.message) for w in dep]}",
+        )
+
+    def test_canonical_webui_testclient_import(self) -> None:
+        with warnings.catch_warnings(record=True) as caught:
+            warnings.simplefilter("always")
+            from webui_testclient import TestClient
+            from webui.app import create_app
+
+            client = TestClient(create_app())
+            response = client.get("/")
+            self.assertIn(response.status_code, {200, 302, 404})
+
+        dep = [
+            w
+            for w in caught
+            if "httpx" in str(w.message).lower()
+            and "deprecated" in str(w.message).lower()
+        ]
+        self.assertEqual(dep, [], [str(w.message) for w in dep])
+
+
+if __name__ == "__main__":
+    unittest.main()