fix: reject resume requests for stale sessions

[brongan]

Summary

Reject resume key updates when any of the following is true:

• Shutdown has started.
• The existing session is not Active.
• An Active session has no key sender.

This is a narrow follow-up to #103. That PR added support for reconnecting to an active stream, but /resume currently treats any existing session object as resumable. The shutdown signal can fire before the watchdog clears that object, leaving a short window where Moonshine acknowledges a resume for a stream that can no longer produce frames.

Resume decision

Before this change, any existing session returned status 200. A missing key sender only logged a warning.

flowchart TD
    A["Receive /resume with new keys"] --> B{"Shutdown started?"}

    B -- Yes --> X["Return status code 400"]
    B -- No --> C{"Session is Active?"}

    C -- No --> X
    C -- Yes --> D{"Key sender exists?"}

    D -- No --> X
    D -- Yes --> E["Replace session keys"]
    E --> Y["Return status code 200"]

Loading

The original failure was observed during a Moonlight reconnect: Moonshine logged ControlStreamStopped, accepted /resume with status 200 during teardown, and then produced no video before Moonlight's first-frame timeout.