Fixes #53981

Selection mentions inserted from the agent panel are stored as folded
placeholder text in the message editor. That meant a normal copy would
put the literal `selection` token on the clipboard, and pasting into a
new thread would lose the original mention context.

This teaches `MessageEditor` to serialize selected folded mentions as
their mention links before writing to the clipboard. The surrounding
text stays intact, and the existing paste path can rebuild the mentions
in a fresh thread.

Release Notes:

- Fixed copied agent prompts replacing selection mentions with the
literal word `selection`.

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Co-authored-by: Neel <neel@zed.dev>