Track ACP model selection method compatibility

[neubig]

Track the bug fix in #3688.

Codex ACP conversations can fail with Method not found because model selection is routed through the older session/set_model RPC. Current Codex and Claude ACP providers expose model selection through session/set_config_option with configId="model", while Gemini still uses the older session model method.

The PR updates provider metadata and model-selection routing so known ACP providers use the correct RPC and includes focused SDK and agent-server regression coverage.

[all-hands-bot]

[Automatic Post]: This issue has been waiting for triage. @xingyaoww @neubig @enyst, could you please take a look and add the appropriate priority label when you have a chance?

This comment was created by an AI agent (OpenHands) on behalf of the user.