Add turn trigger metadata (#40665)

## What changed

- Add an optional `turnTrigger` field to app-server `turn/start` requests and
  expose it in the generated protocol schemas.
- Propagate non-empty trigger values to Responses request metadata as the
  reserved `turn_trigger` field, while preserving the original value when a
  request steers an active turn.
- Classify turns started by queue dispatch, goal continuation, retry recovery,
  and realtime handoff.

## Testing

- Cover HTTP and WebSocket metadata forwarding, steering behavior, reserved
  metadata handling, and the built-in trigger classifications.

GitOrigin-RevId: c12fe2c522286db21b76081ae6154fbf6bfb3639