docs: add AI policy for contributions

[Lancetnik]

Summary

• Adds .github/AI_POLICY.md establishing guidelines for AI usage in contributions
• Adapted from Astral's AI policy under MIT license
• Covers expectations for issues, PRs, and communication with maintainers

[qingyun-wu]

@VasiliyRad could you make sure to review! Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 25 files with indirect coverage changes

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