fix(mu4e-attachments): reject stale MIME handles at save time

The selection buffer captures MIME handles when it opens, but viewing another message kills the buffer a handle points at. Saving through such a handle died deep in mm-decode, or could write another message's bytes. The save path now signals a user-error naming the attachment when its handle's buffer is dead, and it checks before any mu4e MIME support loads, in the same spot as the existing no-handle check.

I added tests for the stale-handle error, the healthy live-handle save, and the mid-batch failure behavior (the error propagates and files already written stay).

0 files changed, 0 insertions, 0 deletions