fix: pin httpx < 0.28 and deduplicate starlette constraint

starlette 0.29 passes app= to httpx.Client.__init__() which was
removed in httpx 0.28, breaking all middleware tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: deborahjacob-botanu

pyproject.toml

@@ -144,10 +144,8 @@ dev = [
     "pytest-asyncio >= 0.21.0",
     "pytest-cov >= 4.1.0",
     "coverage[toml] >= 7.0",
-    "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'",
+    "httpx >= 0.24.0, < 0.28.0",
+    "starlette >= 0.27.0, < 0.30.0",
     "ruff >= 0.4.0",
     "mypy >= 1.7.0",
     "pre-commit >= 3.5.0",