Fixing a crash when exiting or re-entering play mode

[Codengineer]

Fixes UUM-135678. The crash occurs because Academy.HandleOnPlayModeChanged only handles ExitingEditMode, not ExitingPlayMode. This means when you exit play mode:

1. Academy.Dispose() is never called
2. The Worker (inference engine) with its native memory (CPUTensorData containing JobHandle fences) is never properly disposed

Proposed change(s)

Added ExitingPlayMode handling so the Academy properly disposes all ModelRunner instances (and their Worker objects with native memory) before domain reload occurs. This ensures Academy.Dispose() → ModelRunner.Dispose() → Worker.Dispose() → CPUTensorData.Dispose() all run on the main thread during the ExitingPlayMode phase, before domain reload triggers GC finalization on background threads.

Retested intensively enter-exit play mode cycle and could not get it to crash again while it was crashing almost all the times before.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

https://jira.unity3d.com/browse/UUM-135678?filter=-1

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments