[Bugfix] Clean distributed state after worker initialization failure

[AndreasKaratzas]

Summary

• Tear down model-parallel and distributed state when a spawned worker fails before WorkerProc construction completes.
• Add a focused regression for cleanup order on the partial-initialization path.

With spawn, constructor failure can occur after global groups and their shared-memory queue are initialized but before a WorkerProc exists to own normal shutdown.

Validation

• Focused worker cleanup regression: 1 passed.
• Two-GPU invalid Tensorizer shard path: 1 passed in 24.89 seconds, with no surviving worker or GPU allocation in the lifecycle control.
• Changed-file pre-commit passed.

Buildkite: Model Executor

Duplicate check: #34816 handles parent-death signals, not constructor failure; no open PR addresses this ownership path.

AI assistance: Codex assisted with investigation, implementation, and validation; Andreas directed the ownership design and reviewed the resulting changes.