#38646 feat: allow clients to accept calls before the session is established

Proposed changes (including videos or screenshots)

Currently the call can only be accepted from a websocket connection, but in order to implement mobile support the app needs to be able to accept the call from a push notification, before a websocket connection is established. This PR adds an endpoint for that.

Issue(s)

VMUX-35

Steps to test or reproduce

Further comments

Summary by CodeRabbit

• New Features

  • REST endpoint to answer media calls (accept/reject/ack/unavailable) with validation and explicit error responses.
  • New call notifications: active and hangup.

• Improvements

  • Server-side support to answer calls even without an active media session and improved callee-scoped call lookup.
  • Stricter signal processing with optional strict handling, better contract/state validation, and clearer error logging.