[mime] Fix hanging part streams on source stream error or completion

[iv-re]

Fixes #2428

Propagate source stream errors and premature close events (MimeMultipartException) to the active part stream (_multipartController) and close it. This prevents listeners of the active part from hanging indefinitely.

Adds corresponding tests in mime_multipart_transformer_test.dart.

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Code Review

This pull request updates BoundMultipartStream to properly handle errors and premature onDone events from the source stream, preventing hangs by ensuring the active multipart controller is closed and notified. It also adds comprehensive tests to verify this behavior. The reviewer identified a potential bug where a null assertion failure could occur if the source stream continues to emit data after an error, and suggested setting _state = _failCode in the onError handler to prevent further parsing.

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Package	Version	Status	Publish tag (post-merge)
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[lrhn]

No opinion on whether this is a good change, I don't know the code well enough. Just nit-picking on style.

[lrhn]

No _tryPropagateControllerState(); here, like in line 146?

Is it not needed when _controller is being closed immediately after?

[iv-re]

_tryPropagateControllerState() syncs the main controller's state (pause/resume/cancel) to _subscription.

In onDone, the source stream is already finished and _subscription is dead, making any synchronization or cancellation calls on it redundant.