Move AF2 standalone patches to airflowrt pkg

[pritt20]

Description

• Moves AF2 macOS standalone patches (Darwin shim, pickle fix plugin, fork-safety .pth files) and the WriteDarwinForkSafetyPatch function from airflow/ into pkg/airflowrt/, making them importable by astro-cli
  and other apps
• Updates airflow/standalone.go to reference the relocated exports via package-level variable aliases.
• Enhances CheckHealth to probe both /api/v2/monitor/health and /health for AF2, supporting both health endpoint.

🎟 Issue(s)

Related #XXX

🧪 Functional Testing

Tested change locally:

• Verify astro dev start works for AF2 on macOS (Darwin shim + fork-safety patch applied correctly)
• Verify astro dev start works for AF3 (no behaviour change)

📸 Screenshots

Add screenshots to illustrate the validity of these changes.

📋 Checklist

• Rebased from the main (or release if patching) branch (before testing)
• Ran make test before taking out of draft
• Ran make lint before taking out of draft
• Added/updated applicable tests
• Tested against Astro-API (if necessary).
• Tested against Houston-API and Astronomer (if necessary).
• Communicated to/tagged owners of respective clients potentially impacted by these changes.
• Updated any related documentation