Allow per-call sideband endpoints for existing realtime calls

[copyberry]

Allow per-call sideband endpoints for existing realtime calls

What changed

• Add an in-process sideband_base_url override to ConversationStartTransport::ExistingCall, while keeping app-server requests on the configured or default endpoint.
• Prefer the per-call endpoint when attaching and reconnecting a sideband, falling back to experimental_realtime_ws_base_url when no override is supplied.
• Preserve runtime authentication headers on the selected endpoint without adding bearer authorization.

Testing

• Cover configured and per-call endpoint selection, authentication headers, call-specific handshake paths, and reconnect behavior.

[github-actions]

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