feat(librarian): allow update-image to be a no-op

[jskeet]

The update-image command is really useful for container authors, in terms of checking performance, behavior, and configuration. It's useful to be able to run it even without a new image - because if everything is in a steady state, we shouldn't see any changes at all.

We still log that the image is the same, but don't count it as an error.

[gemini-code-assist]

Summary of Changes

Hello @jskeet, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the update-image command within the librarian service by allowing it to complete its execution even when the target image is the same as the currently deployed image. This modification enables container authors to perform diagnostic runs, such as checking performance or configuration, without requiring an actual image change. The system will now log that no image update was necessary but will not treat this as an error or terminate the process prematurely.

Highlights

• Update Image Command Behavior: The update-image command in internal/librarian/update_image.go no longer aborts its execution when the current image is identical to the desired image. Instead, it now logs this condition as a diagnostic run and proceeds with the remaining steps.
• Test Case Adjustment: The corresponding test case in internal/librarian/update_image_test.go has been updated to reflect this new behavior. Specifically, wantGenerateCalls is now expected to be 1 (previously 0) and wantCheckoutCalls is now expected to be 2 (previously 0) for scenarios where no image update occurs.

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[gemini-code-assist]

Code Review

This pull request modifies the update-image command to allow it to run even when there is no new container image, which is useful for diagnostic purposes. The change correctly removes the early exit when the image is unchanged, allowing the regeneration process to proceed. The corresponding tests have been updated to reflect this new behavior, ensuring that regeneration is now expected and verified in the no-change scenario. The implementation is straightforward and correct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.50%. Comparing base (2f8457f) to head (7241054).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2807      +/-   ##
==========================================
- Coverage   86.50%   86.50%   -0.01%     
==========================================
  Files         120      120              
  Lines       10232    10231       -1     
==========================================
- Hits         8851     8850       -1     
  Misses        977      977              
  Partials      404      404              

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