feat(bot-mode): share files through Group Chats

[dokterdok]

The user problem

Sharing a file with a Bot Group Chat should feel like sharing one in any normal
chat. It should not matter whether the chat is hosted by a gateway, still run by
Desktop, or reached from Signal, Telegram, WhatsApp, Slack, or another Hermes
messaging adapter.

Today those paths do not share one complete lifecycle. This draft connects the
existing Desktop attachment UX to the durable gateway layer from the parent PR.

Important

This draft is stacked on the attachment runtime PR. Review
57962642d7..a7eddf11a3.

What users can do

From Desktop

Use the existing picker, paste, or drag and drop in a Group Chat. Images, PDFs,
Office files, archives, and other files are uploaded before the durable room
command is accepted.

From a messaging app

Attach a file to the normal Group Chat command:

/group list
/group 2
/group 2 send

The attachment may be sent with text or by itself. Hermes localizes it through
the adapter's authenticated media cache, stores it once, and reports the same
Group Chat status and recent activity as a text message.

In history

History keeps the file name, type, size, opaque ID, and source gateway, never
the raw bytes or a machine path. Images can load a preview on demand; files can
be downloaded on demand. The local cache is bounded.

What users can rely on

• The draft stays in the composer until every upload and the durable command
  enqueue succeed.
• Typing during an upload preserves the newer text and newly added files while
  removing only the files that were actually sent.
• Transport redelivery reuses the same upload and room entry, even after the
  adapter's temporary file has disappeared.
• A Desktop command reads files only while it holds the live room claim.
• The Bot roster is frozen at send time. A Bot added later cannot receive an
  older file, and an unavailable original Bot keeps the command safely pending.
• A Stop request can overtake a long upload/read and no partial file reaches a
  Bot turn.
• One bad or expired file produces an actionable error without exposing paths,
  URLs, bytes, or provider details.

Compatibility

This reuses the attachment controls and per-session image/PDF/file RPCs merged
in #89486 and #89540. It preserves opaque IDs so the open chip interaction in
#89758 can compose with gateway-backed history; if #89758 lands first, this
draft should rebase onto its final chip implementation rather than duplicate it.

Boundaries

• This stack completes same-gateway, Desktop-driven, and messaging-client file
  flows.
• Cross-gateway RoomLink byte transfer remains the next stacked increment.
• Messaging adapters must provide authenticated, already-localized cache paths,
  which is the existing MessageEvent.media_urls contract. Remote URLs and
  symlinks fail closed.
• Unsolicited notifications and public file URLs are not added.

Validation at a7eddf11a3

Check	Result
Focused Python suite	242 passed
Focused Desktop/hosted-client suite	165 passed
Desktop TypeScript check	passed
Ruff	passed
git diff --check	passed

Coverage includes image-only and attachment-only sends, PDF and generic files,
adapter redelivery, upload interruption, lease loss, priority Stop, send-time
recipient freeze, restart/replay, lazy history reads, tampering, MIME mismatch,
path traversal, symlinks, quotas, abandoned-upload cleanup, and composer edits
during upload.

Exact-head live UAT and screenshots will be added before this stack leaves
draft.

Related work

• feat(desktop): group-chat bots now see user-attached images #89486: merged image attachment pipeline for Desktop Group Chats
• feat(desktop): group chats gain PDF/file attachments and drag & drop (#88983) #89540: merged PDF/file and drag/drop parity
• feat(desktop): group-chat attachment chips open in the file manager #89758: open attachment-chip interaction follow-up
• [Feature]: Bots group chat — support file uploads (same as individual bot chats) #88983: original Group Chat attachment parity request
• feat(bot-mode): control hosted rooms from messaging chats #96274 / feat(bot-mode): control Desktop-driven Group Chats from messaging #96654: messaging control across hosted and Desktop-driven chats

Type of change

• Bug fix
• New feature
• Security fix
• Tests
• Refactor

[dokterdok]

Closing this cumulative prototype now that #97681 captures the user contract, safety boundaries, and phased landing plan against the rebuilt Bot Mode tree. Its useful layer can be re-cut as a narrow current-main contribution after the foundation direction is agreed.