Add per-session realtime model and version overrides

[guinness-oai]

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

[guinness-oai]

@codex