fix: pr size labeler should keep the label if appropriate

[mathusanm6]

No description provided.

[Copilot]

Pull Request Overview

This PR fixes the PR size labeler workflow to avoid unnecessary label churn by keeping the existing size label when it matches the calculated size. The change replaces the third-party codelytv/pr-size-labeler action with a custom implementation that calculates PR size and manages labels more intelligently.

• Implements custom PR size calculation logic to determine appropriate size labels
• Only removes existing size labels when they differ from the calculated label
• Adds separate step to handle XL PR warning comments with duplicate prevention

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