feat: add roundRobinHostId to booking creation API and mapping

[KartikLabhshetwar]

What does this PR do?

• Fixes #XXXX (GitHub issue number)
• Fixes CAL-XXXX (Linear issue number - should be visible at the bottom of the GitHub issue description)

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Summary by cubic

Adds support for pre-selecting a specific host for round-robin events via a new roundRobinHostId field. When provided, availability and booking target only that host; when omitted, existing round-robin logic remains.

• New Features
  • Added optional roundRobinHostId to booking creation body and API DTOs.
  • Accepted in slots queries; availability is limited to that host when set.
  • Booking and slots services validate the host is eligible; return an error if not available for the event.
  • Booker Platform adds a roundRobinHostId prop and forwards it through hooks and the booking mapper.
  • Updated zod/TRPC/validation schemas and controllers to pass the field end-to-end.

^{Written for commit b3bf3fb3b15edd014d8fb0091d9543764a0ca54f. Summary will update automatically on new commits.}

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