Fix SFTP pattern timestamp comparisons with non-UTC settings

[mifuha]

When SFTPTrigger uses file_pattern with newer_than, different process and Airflow timezones can make it choose the wrong files. A file modified at 12:00 UTC can incorrectly pass a 14:00 UTC threshold. Reversing the timezone settings can reject a file that is new enough.

This fix reads the modification timestamp directly as UTC. It keeps the existing inclusive comparison and whole-second precision.

The regression tests run the real trigger in fresh processes with different timezone settings and mocked SFTP calls. They cover both wrong decisions, UTC controls, equal timestamps, a non-hour timezone offset, fractional seconds and missing modification times.

Related: #73382. The single-file trigger and synchronous sensor paths still use the existing string-based hook methods and remain affected. This PR leaves those interfaces unchanged, so the issue should stay open.

Checks run with locked dependencies:

• Both mismatch cases failed before the fix; the UTC controls passed.
• All 55 focused SFTP tests passed after the fix on Python 3.12.3.
• All 11 timezone regression cases passed on Python 3.10.21 and 3.12.3.
• The regular pre-commit checks and commit-message check passed.
• git diff --check passed.

The manual provider check could not finish because the local environment reported bus and I/O errors while Docker was exporting its CI image. The full provider suite, live SFTP tests and compatibility matrix were not run locally.

— @mifuha

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

• Yes — Codex (GPT-6)

AI-assisted: following the guidelines. I used Codex to help investigate the issue, write the fix and tests.

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