chore(ci): fix header comment loss in update_overrides on repeated runs

[yogarajalakshmi-s]

What this PR does / why we need it:
When has_overrides=True, update_overrides.py removes the "# Security overrides" header comment but never re-adds it. On the next run, the insertion logic uses this header as a bookmark to know where to insert the override block — but since it was deleted, the block gets inserted incorrectly. Fixed by reinserting the header after [tool.uv] following the cleanup step.

This bug was discovered while adapting this script for kubeflow/trainer in kubeflow/trainer#3539.

Which issue(s) this PR fixes:
Fixes #502

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates the update_overrides.py helper to keep the pyproject.toml “Security overrides” header stable when regenerating override-dependencies, and applies small formatting cleanups.

Changes:

• Reformats long print(...) and re.sub(...) calls for readability.
• Removes existing override-dependencies blocks and associated header comments.
• Re-inserts a standardized “Security overrides” header after [tool.uv] to support consistent insertion behavior.

[yogarajalakshmi-s]

Hi @Fiona-Waters! Could you review this PR?

[Fiona-Waters]

Thanks for this @yogarajalakshmi-s
Looks good. Should we include \r here too?

[andreyvelich]

/ok-to-test
/retest

[yogarajalakshmi-s]

Should we include \r here too?

Yes! Updated both insertion points to use \r

[Fiona-Waters]

Thanks @yogarajalakshmi-s
/lgtm