Fix incompat of request.form with anyio 4.14.0 in aws auth manager

[amoghrajesh]

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

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.

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.