feat(gateway): native send_multiple_images for Telegram, Discord, Slack, Mattermost, Email

Ports PR #17888's send_multiple_images ABC to every gateway platform that
has a native multi-attachment API, so images arrive as a single bundled
message instead of N separate ones.

Native overrides:
- Telegram: send_media_group (10 photos per album, chunks over); animated
  GIFs peeled off and routed through send_animation (albums don't support
  animations)
- Discord: channel.send(files=[...]) (10 attachments per message, chunks
  over); URL images downloaded into BytesIO so they render inline; forum
  channels use create_thread with files=[...]
- Slack: files_upload_v2(file_uploads=[...]) (10 per call, chunks over);
  respects thread_ts; records thread participation
- Mattermost: single post with file_ids list (5 per post — Mattermost cap,
  chunks over)
- Email: single SMTP message with multiple MIME attachments (no chunk cap,
  SMTP size governs); remote URLs remain linked in body (parity with
  existing send_image)

All platforms fall back to the base per-image loop on any failure, so a
single bad image in a batch never loses the rest.

Matrix, WhatsApp, and single-attachment platforms (BlueBubbles, Feishu,
WeCom, WeChat, DingTalk) continue to use the base default loop — their
server APIs only accept one attachment per message anyway.

Tests: adds tests/gateway/test_send_multiple_images.py with 19 targeted
tests covering base default loop, chunking, animation peel-off, fallback
paths, and empty-batch no-ops across all five new overrides.

Co-authored-by: Maxence Groine <maxence@groine.fr>

scripts/release.py

@@ -58,6 +58,7 @@ AUTHOR_MAP = {
     "nbot@liizfq.top": "liizfq",
     "274096618+hermes-agent-dhabibi@users.noreply.github.com": "dhabibi",
     "dejie.guo@gmail.com": "JayGwod",
+    "maxence@groine.fr": "MaxyMoos",
     # OpenViking viking_read salvage (April 2026)
     "hitesh@gmail.com": "htsh",
     "pty819@outlook.com": "pty819",