Fix Matrix-Game-3 pipeline exit handling

[Arison591]

Summary

• return the generated video from both Matrix-Game-3 pipeline variants
• leave distributed-process-group cleanup to the top-level caller that initialized it
• allow generate.py to run its synchronization, cleanup, and final log statements

Root cause and impact

Both MatrixGame3Pipeline.generate implementations called exit() after saving the output. This terminated the entire interpreter instead of returning control to the caller, so the final statements in generate.py were unreachable and the pipelines could not be embedded in another Python process. They also destroyed the process group internally even though generate.py already owns that lifecycle.

Closes #77.

Validation

• python -m compileall -q Matrix-Game-3/generate.py Matrix-Game-3/pipeline/inference_pipeline.py Matrix-Game-3/pipeline/inference_interactive_pipeline.py
• python -m ruff check Matrix-Game-3/generate.py Matrix-Game-3/pipeline/inference_pipeline.py Matrix-Game-3/pipeline/inference_interactive_pipeline.py --select F821,F822,F823
• RTX 5090 CUDA regression with stubbed model components and zero generation iterations, confirming that both non-interactive and interactive generate() methods return normally instead of raising SystemExit