## Why
Clients need to select a realtime session configuration for an
individual start without rewriting persisted configuration or restarting
the app-server process.
## What Changed
- Add optional `model` and `version` fields to `thread/realtime/start`
- Forward those optional values through the realtime start operation and
apply them only for that session
- Preserve existing configured/default behavior when the new fields are
omitted
- Update generated protocol schema and app-server documentation
## Validation
- Added/updated protocol serialization coverage for the new optional
request fields
- Added focused core coverage for a session override taking precedence
over configured realtime selection
- Added focused app-server coverage that a request override reaches the
realtime WebSocket handshake
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