chore: catch up several changes from oneshot crate

[tisonkun]

Still lack for reviewing other changes, like:

• faern/oneshot@acee12c

[Copilot]

Pull request overview

This PR syncs a couple of upstream oneshot-crate changes into mea’s oneshot implementation, including small internal API adjustments and additional regression tests around sender/receiver lifecycle and message availability.

Changes:

• Add tests verifying receiver closure when the sender is dropped and has_message() behavior.
• Refine internal channel message access (Channel::message) to return &T and update SendError::as_inner accordingly.
• Add/expand safety commentary and adjust SendError<T> debug output to use type_name::<T>().

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
mea/src/oneshot/tests.rs	Adds new tests for sender-drop closure propagation and has_message() semantics.
mea/src/oneshot/mod.rs	Adjusts internal message accessor and SendError impls/comments; adds Receiver<T>: Unpin impl and tweaks Debug.

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