## What changed
Add optional `voiceSessionId` to realtime start parameters and forward it as the `voice-session-id` header for WebSocket, WebRTC, and existing-call transports. Keep this client-generated voice session identifier distinct from `realtimeSessionId`, which uses `x-session-id`.
## Testing
Extend serialization tests and add coverage for distinct session ID headers, WebRTC request forwarding, and existing-call handshake forwarding.
GitOrigin-RevId: cb558dbb9d438f2d34e281199a7e81fe5d33b879
## Why
A delayed detach or cleanup from an older realtime conversation must not close its replacement or end the replacement's history segment. Realtime start parameters can also contain credentials in text and transport endpoints, so diagnostics should avoid exposing them.
## What changed
- Add `RealtimeConversationAttach` to report the connection attempt's result through a reply channel, using the sequential start path.
- Add `RealtimeConversationDetach` to close only the matching `realtime_session_id` after earlier starts finish. Detaching an older session leaves the replacement connected.
- Emit the previous conversation's close event before starting its replacement, and prevent stale fanout cleanup from closing or clearing the replacement's context.
- Limit `ConversationStartParams` debug output to output modality, initial item count, and protocol version.
## Testing
Add coverage for attachment completion after the handshake, stale detaches, failed replacement and retry, natural-close ordering, handoff history after replacement, and debug redaction for existing-call and WebRTC transports.
GitOrigin-RevId: 23aee6f70522f0b17e6aa9349dbbcc2ea985f233
Chris Dong