Fix MissingReferenceException when stopping scene with active mic

[MaxHeimbrock]

Problem

Stopping the scene with an active microphone source consistently throws:

MissingReferenceException: The object of type 'LiveKit.MonoBehaviourContext' has been destroyed but you are still trying to access it.
  at LiveKit.MonoBehaviourContext.RunCoroutine (MonoBehaviourContext.cs:39)
  at LiveKit.MicrophoneSource.Stop (MicrophoneSource.cs:148)
  at MeetManager.OnDestroy ...

On play-mode exit, Unity tears down the DontDestroyOnLoad LiveKitSDK GameObject (backing the MonoBehaviourContext singleton) before MeetManager.OnDestroy() runs. MicrophoneSource.Stop() then calls RunCoroutine, which did _instance.StartCoroutine(...) on the destroyed instance and threw.

Fix

Guard RunCoroutine: when _instance has been destroyed (Unity's overloaded == reports it as null), drain the coroutine synchronously via a new DrainCoroutine helper and invoke onComplete, instead of touching the dead MonoBehaviour. DrainCoroutine recurses into nested enumerators and skips time-based yields (meaningless during shutdown). This keeps cleanup side effects working and protects every caller — MicrophoneSource is currently the only one, but the fix lives at the shared entry point rather than at the call site.

Test

Added an EditMode regression test (Tests/EditMode/MonoBehaviourContextTests.cs) that deterministically reproduces the failure mode: it points _instance at a DestroyImmediate-d object and asserts RunCoroutine drains without throwing and still runs the body + onComplete. The original singleton is saved/restored via reflection so other tests are unaffected.

The exception fires synchronously from StartCoroutine once its host is destroyed, so the test reproduces the precondition deterministically without needing Unity's actual play-mode-exit teardown ordering. Against the pre-fix code it fails with MissingReferenceException; with the fix it passes.