chore: bump anyio minimum to 4.9 for free_tcp_port fixture

The free_tcp_port pytest fixture was added in anyio 4.9.0. The
lowest-direct CI matrix was resolving to 4.5.0, which doesn't
have it. Locked version was already 4.10.0 so no lockfile churn
beyond the constraint metadata.

Author: maxisbey

pyproject.toml

@@ -25,7 +25,7 @@ classifiers = [
     "Programming Language :: Python :: 3.14",
 ]
 dependencies = [
-    "anyio>=4.5",
+    "anyio>=4.9",
     "httpx>=0.27.1",
     "httpx-sse>=0.4",
     "pydantic>=2.12.0",