Add CopilotAPI service for a future Claude agent (#313553)

* Add CopilotAPI service & plan for Claude

This service can be used to send requests to CAPI which we will need as we shim requests from agents to CAPI.

* Add CopilotAPI service & plan for Claude

This service can be used to send requests to CAPI which we will need as we shim requests from agents to CAPI.

* Clarify signal propagation behavior in ICopilotApiServiceRequestOptions documentation