[remote-controlled-man]

Found while adding setup-error coverage for fastai/fastai#4186. If an early manager acquires a lock and a later manager raises in __enter__, ContextManagers.__enter__ exits without unwinding its ExitStack. The lock stays acquired while the wrapper or traceback is still reachable, so subsequent work can block.

Unwind the already-entered stack with the exception information before propagating the entry failure. Add regression coverage using a real lock and temporary environment state, plus reverse exit order, identical RuntimeError/KeyboardInterrupt objects, later managers/body not entered, successful entry and empty collections. The notebook documents the failure behavior.

Validation on macOS, Python 3.12:

• The lock/environment regression fails on unchanged main and passes with this fix.
• Complete native nbdev-test --n-workers 2 suite: passed, with no notebook selection or added skips.
• Complete 03_xtras.ipynb test, nbdev-clean, source export and clean/diff checks: passed.

The other OS/Python combinations in the CI matrix were not run locally.

AI assistance: OpenAI Codex assisted implementation, tests and review.

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