feat(send_message): add media delivery support for Signal

Cherry-picked from PR #13159 by @cdanis.

Adds native media attachment delivery to Signal via signal-cli JSON-RPC
attachments param. Signal messages with media now follow the same
early-return pattern as Telegram/Discord/Matrix — attachments are sent
only with the last chunk to avoid duplicates.

Follow-up fixes on top of the original PR:
- Moved Signal into its own early-return block above the restriction
  check (matches Telegram/Discord/Matrix pattern)
- Fixed media_files being sent on every chunk in the generic loop
- Restored restriction/warning guards to simple form (Signal exits early)
- Fixed non-hermetic test writing to /tmp instead of tmp_path

scripts/release.py

@@ -97,6 +97,7 @@ AUTHOR_MAP = {
     "ahmedsherif95@gmail.com": "asheriif",
     "liujinkun@bytedance.com": "liujinkun2025",
     "dmayhem93@gmail.com": "dmahan93",
+    "cdanis@gmail.com": "cdanis",
     "samherring99@gmail.com": "samherring99",
     "desaiaum08@gmail.com": "Aum08Desai",
     "shannon.sands.1979@gmail.com": "shannonsands",