fix: cap httpx < 0.28 for Python 3.9 (starlette TestClient compat)

httpx 0.28+ removed the `app` parameter from Client.__init__(),
breaking starlette < 0.30's TestClient. Cap httpx on Python 3.9
where starlette is pinned to < 0.30.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Deborah Jacob <deborah@botanu.ai>

Author: deborahjacob-botanu

pyproject.toml

@@ -131,7 +131,8 @@ dev = [
     "pytest-asyncio >= 0.21.0",
     "pytest-cov >= 4.1.0",
     "coverage[toml] >= 7.0",
-    "httpx >= 0.24.0",
+    "httpx >= 0.24.0, < 0.28.0; python_version < '3.10'",
+    "httpx >= 0.24.0; python_version >= '3.10'",
     "starlette >= 0.27.0, < 0.30.0; python_version < '3.10'",
     "starlette >= 0.27.0; python_version >= '3.10'",
     "ruff >= 0.4.0",