Add AI contribution guidelines to PR checklist

[stuartmorgan-g]

Updates the PR checklist to included an item about AI expectations, matching flutter/flutter#183326

[gemini-code-assist]

Code Review

This pull request updates the pull request template file. It adds a new item to the pre-review checklist for contributors to acknowledge the AI contribution guidelines. A corresponding link definition pointing to the guidelines in the main Flutter repository is also added.

_{Note: Security Review has been skipped due to the limited scope of the PR.}