feat(realtime): support sideband call_id connections

[felmonon]

Summary

Add callID support to the stable and beta Realtime WebSocket helpers for the documented OpenAI sideband control flow.

This updates the shared Realtime URL builder and both ws and browser WebSocket helpers to accept exactly one of:

• model to start a new Realtime session
• callID to attach to an in-progress OpenAI Realtime call via ?call_id=

For safety, Azure helpers remain deployment-based and reject callID explicitly rather than implying unsupported behavior.

Closes #1747.

Changes

• add a shared RealtimeConnectionConfig with a runtime guard for model xor callID
• thread callID support through stable and beta Realtime helpers
• document the sideband callID flow in realtime.md
• add focused tests covering OpenAI and Azure URL construction and invalid config cases

Testing

• ./node_modules/.bin/jest tests/realtime.test.ts
• ./node_modules/.bin/eslint src/realtime/internal-base.ts src/realtime/ws.ts src/realtime/websocket.ts src/beta/realtime/internal-base.ts src/beta/realtime/ws.ts src/beta/realtime/websocket.ts tests/realtime.test.ts
• yarn build