Steer active background Responses

[m5i-work]

• Make sure you've installed the latest version using instructions

Output from azd version
N/A - implementation tracking issue.

Describe the bug
Implement the third delivery slice of #9676: message-bearing --continue that reuses the saved conversation and compatible hosted session to steer active work or start the next background turn.

To Reproduce
N/A - feature implementation.

Expected behavior
Users can submit revised input to active work without forking conversation history, and use the same command to start the next turn after completion.

Environment
N/A - Azure AI Agents extension.

Additional context
Parent issue: #9676
Implementation PR: #9705
This is one scoped delivery slice of the parent issue.

[m5i-work]

Closing as superseded by the revised #9676 design. Steering requires no dedicated CLI implementation: another ordinary Response create using the same conversation lets the service apply steering behavior. The active-Response guard is removed in #9900.