Close the ssh askpass helper before ssh executes it

[MichalJaroslawKrzywanski-TomTom]

GitHook writes the SSH_ASKPASS helper that unlocks a passphrase-protected private key with NamedTemporaryFile(delete=True) and keeps the handle open for writing while ssh runs. Linux refuses to exec a file that is still open for writing (ETXTBSY, "Text file busy"), so a clone or fetch with a passphrase-protected key fails with cannot exec '/tmp/tmpXXXX.sh': Text file busy and then falls back to prompting for the passphrase. macOS does not enforce this, which hid the bug.

The fix writes the helper through a small module-level context manager (_executable_script) that closes the file before yielding its path and unlinks it in finally.

Scope changed after a rebase on main

This PR originally fixed the same bug on both askpass paths. #64105 has since replaced GIT_ASKPASS with a credential helper that is written and closed before git runs, which fixes the token path upstream, so only SSH_ASKPASS is left to fix here. The test that exercised the old GitHub App askpass script is dropped with the mechanism it tested; #64105's own tests cover the credential helper.

That also means #73425, which reported the failure on the GitHub App path, is already fixed on main by #64105 rather than by this PR — hence related: instead of closes:.

Verification

Run in apache/airflow:3.2.2-python3.10 (Debian 12, git 2.39.5) against the rebased branch:

providers/git/tests/unit/git/hooks/test_git.py .......................................................
55 passed

With main's unpatched hook, the added test fails there:

E   OSError: [Errno 26] Text file busy: '/tmp/tmpyrbc4wtp.sh'

ruff check and ruff format --check are clean on both changed files.

related: #73425
related: #64105

---

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

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Generated-by: Claude Code following the guidelines. The diagnosis, the fix, the tests and the rebase resolution were reviewed by hand; the ETXTBSY reproduction and the test runs above were executed by me.

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[shahar1]

Thanks!

[shahar1]

@MichalJaroslawKrzywanski-TomTom Please do not merge from main so I could see that all checks are green and merge. Thank you!