add a draft gemini review check

[derrickaw]

Problem: When opening a draft PR and the user happens to request a /gemini review and then later on makes the PR ready for review, another gemini review occurs and possibly duplicates the previous findings, rebuttals etc. See #40587 for context.
Plan: add a check to the gemini review workflow to prevent duplicate reviews when switching from draft to ready for review states.

[github-actions]

Assigning reviewers:

R: @damccorm for label build.
Backup reviewer: Amar3tto (if the main reviewer is unresponsive, you can escalate to them by tagging their username)

Note: If you would like to opt out of this review, comment assign to next reviewer.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[github-actions]

📋 Review Summary

This PR adds a pre-check step to the Gemini code review workflow to skip automatic reviews on the ready_for_review event if a Gemini review has already been posted on the PR (e.g., via /gemini review while in draft). The implementation cleanly reuses actions/github-script@v9 and properly gates the subsequent workflow steps.

🔍 General Feedback

• The pagination over github.rest.pulls.listReviews and matching against github-actions[bot] and the /gemini review footer signature accurately identifies prior reviews from this workflow.
• The conditional steps.check_draft_review.outputs.reviewed_in_draft != 'true' behaves as expected both when check_draft_review runs and when it is skipped on other trigger events.

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💡 Tip: Comment /gemini review on this pull request to trigger a new Gemini code review.

[derrickaw]

Verification that it works - #40594 (comment) - thanks