An agent can overwrite a local image at the same path and embed it in a
new reply. The new reply reused the cached signed URL from the earlier
reply, so the browser showed the previous bytes until a reload.

Key each reply's media-file URL by a client-only cache revision (the
reply's message id). It is stripped before the RPC, so a new reply mints
its own URL while the existing atom TTL bounds the cache.

Fixes #15676

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>