feat(ai): steer background Responses

[Wei Meng (m5i-work)]

Summary

Third vertical slice of #9676, stacked on #9704.

• adds message-bearing --continue
• steers active Responses through previous_response_id
• starts a normal next background turn when the saved Response is terminal
• refreshes service status before enforcing the active-turn guard
• handles an active-to-terminal race with one status refresh and next-turn fallback
• reconnects to an accepted replacement if its create stream disconnects

Validation

• go test ./... in cli/azd/extensions/azure.ai.agents
• go fix ./...
• built and installed with azd x build
• live hosted-agent validation against longrun-responses-v2-ref:1:
  • terminal saved Response created a distinct next background turn and completed
  • controlled active Response with 20 one-second steps was seeded in a clean hosted session
  • azd ai agent invoke <agent> "replacement" --continue created and completed a distinct replacement
  • the original active Response reached cancelled, verified by a service GET

Stack

• Base: feat(ai): reconnect and cancel background Responses #9704
• Parallel sibling: Invocations one-shot retrieval

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).
20 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.