Release Notes:

- Fixed send failure for pasted file#line mention links


Handle `MentionUri::Selection` with a file path in
`confirm_mention_for_uri`
by resolving it to text content via existing line-range loading logic,
instead
of returning "Unsupported mention URI type for paste".

Keep untitled-buffer selections (`abs_path: None`) as an explicit
unsupported
error for now.

Add a regression test for pasting selection mentions to ensure they
resolve to
`Mention::Text` with the expected line range.

Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>