Delete old mlinter review comments before posting new ones (#48107)

* Delete old mlinter review comments before posting new ones

Prevents accumulation of duplicate inline comments when findings change
between CI runs. Extracts the previous review ID from the PR description
state block and deletes only its inline comments before posting a new review.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Guard against RuntimeError when fetching old review comments

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>