ACP: support elicitation for V2 forms and questions

Summary

Add ACP elicitation support in V2 by translating session forms/questions into unstable_createElicitation requests when the connected ACP client advertises support.

Today, all session forms are auto-cancelled in the ACP path, so an agent cannot collect structured input from a capable client.

Expected behavior

• Detect whether the ACP client supports elicitation.
• Translate V2 session forms/questions into unstable_createElicitation requests when supported.
• Return the client's submitted or cancelled result to the waiting session flow.
• Preserve the current auto-cancel fallback when elicitation is unsupported.
• Cover supported, cancelled, and unsupported-client paths with tests.

Requested by: @nexxeln (Shoubhit Dash via Slack)