tests: StarletteDeprecationWarning - TestClient wants httpx2, suite warns on every run

[vdmkenny]

Summary

Every pytest run that touches a TestClient-based test emits:

StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
  from starlette.testclient import TestClient as TestClient  # noqa

It originates from fastapi/testclient.py importing starlette.testclient, which now prefers the httpx2 client. The environment currently has httpx 0.28.1 installed and no httpx2.

Why it matters

Same rationale as #2740 (the SQLAlchemy MovedIn20Warning): a permanently warning test suite trains everyone to ignore the warnings summary, which is exactly where a real deprecation will eventually hide. Starlette will drop the legacy httpx TestClient path at some point and the suite should be ahead of that.

Proposed fix

• Add httpx2 to the test/dev requirements so starlette.testclient picks it up (the runtime httpx usage elsewhere in the app is unaffected and can migrate separately if ever needed).
• Verify the suite runs warning-free for this category afterwards: python -m pytest tests/ -W error::DeprecationWarning -k <testclient-heavy subset> or simply confirm the warnings summary no longer lists it.

Notes

• Tracked separately from Resolve SQLAlchemy 2.0 deprecation warnings in core/database.py #2740, which covers the SQLAlchemy 2.0 deprecations in core/database.py. Together they are the only two recurring warnings in the suite today.