chore: Add cursor rules for AI integrations contributions

[github-actions]

Note

The pull request "chore: Add cursor rules for AI integrations contributions" was created by @RulaKhaled but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

This PR adds .cursor/rules/adding-a-new-ai-integration.mdc, a complete reference guide for implementing AI provider integrations (OpenAI, Anthropic, Vercel AI, LangChain, etc.) in the Sentry JavaScript SDK.

[linear]

JS-1639