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Fixes broken CI as seen in https://github.com/apache/airflow/actions/runs/27941132600/job/82677618222, its due to this diff:

< anyio==4.13.0
---
> anyio==4.14.0

anyio 4.14.0 tightened the type stub for from_thread.run to require a callable returning a plain Coroutine. Starlette's request.form returns AwaitableOrContextManager[FormData] which is awaitable but not a Coroutine, so mypy now rejects it.

Introduce a small _fetch_form async helper that wraps await request.form() so from_thread.run receives a properly-typed coroutine function, without changing the sync handler architecture.

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