Starlette 1.0+ deprecates HTTP_422_UNPROCESSABLE_ENTITY for HTTP_422_UNPROCESSABLE_CONTENT

Per RFC 9110, the 422 reason phrase was renamed from Unprocessable Entity to Unprocessable Content; wire status remains 422. Starlette (verified 1.0.0, 2026-05) keeps the old constant as a deprecated alias that emits a DeprecationWarning on access. Both constants are the same integer, so swapping is a pure name change.

Replace HTTP_422_UNPROCESSABLE_ENTITY with HTTP_422_UNPROCESSABLE_CONTENT in production and tests. Grep before and after, rerun the suite. Check upstream state in starlette/status.py and RFC 9110 section 15.5.21.