Two races in the drop-time eager upload:
- Resurrected chip: the success path used addComposerAttachment, which
re-appends when the id is gone, so a file removed mid-upload reappeared once
the upload resolved. Add updateComposerAttachment (update-only; no-op when the
chip was removed) and use it on both the eager success path and submit-time
sync.
- Duplicate upload: submit-time sync didn't join an eager upload still in
flight, so drop-then-Enter could fire file.attach twice and leave a duplicate
under .hermes/desktop-attachments/. Track in-flight eager uploads by id and
await the pending one before deciding to re-upload, reusing its gateway ref.
Tests: composer-store no-resurrect unit tests + a join-on-submit integration
test asserting a single file.attach.
Addresses @helix4u review on #43109.
891c9a6823