Gate post-commit Ci::Build sticking

What does this MR do and why?

This MR adds a default-disabled feature flag, ci_stick_build_after_commit, to defer sticking running CI builds until after the status update commits.

The disabled path preserves the existing after_save behavior while the change is rolled out. When the flag is enabled, stick_build_if_status_changed schedules the stick with run_after_commit instead of sticking immediately during the save callback.

This avoids moving the behavior unconditionally while we validate whether post-commit sticking is safer for transaction duration and replica catch-up semantics.

References

• Hopefully fixes #596961 or subset thereof
• The after_save hook was first added in !1502 (merged), with no discussion of after_commit.

Screenshots or screen recordings

Not applicable. This is a backend-only change.

How to set up and validate locally

1. Enable the feature flag for a project:

   Feature.enable(:ci_stick_build_after_commit, Project.find(<project_id>))

2. Transition a pending CI build for that project to running.

3. Confirm build sticking happens after the status update commits.

MR acceptance checklist

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