Workspace-side fetcher for the platform-staged chat uploads written by
phase 1. Stack atop feat/poll-mode-chat-upload-phase1.
Wire shape — the platform writes one activity_logs row per uploaded
file with `activity_type=a2a_receive`, `method=chat_upload_receive`,
and a `request_body={file_id, name, mimeType, size, uri}` carrying
the synthetic `platform-pending:<wsid>/<fid>` URI.
Workspace-side flow (new module workspace/inbox_uploads.py):
1. Fetch via GET /workspaces/:id/pending-uploads/:file_id/content
2. Stage to /workspace/.molecule/chat-uploads/<32-hex>-<sanitized>
(same on-disk shape as internal_chat_uploads.py — agent-side
URI resolvers see no contract change)
3. POST /workspaces/:id/pending-uploads/:file_id/ack
4. Cache `platform-pending: → workspace:` so the eventual chat
message that REFERENCES the upload (separate, later activity row)
gets URI-rewritten before the agent sees it.
Inbox poller extension (workspace/inbox.py):
- is_chat_upload_row(row) discriminator on `method`
- upload-receive rows trigger fetch_and_stage and are NOT enqueued
as InboxMessages (they're side-effect rows, not chat messages)
- cursor advances past them regardless of fetch outcome — a
permanent /content failure must not stall the cursor and block
real chat traffic
- message_from_activity calls rewrite_request_body to swap
platform-pending: URIs to local workspace: URIs in subsequent
chat messages' file parts. Cache miss leaves the URI untouched
so the agent surfaces an unresolvable URI rather than the inbox
silently dropping the part.
Filename sanitization mirrors workspace-server/internal/handlers
/chat_files.go::SanitizeFilename and workspace/internal_chat_uploads
.py::sanitize_filename — pinned by the existing parity test suites.
Coverage: 100% on inbox_uploads.py; the inbox.py extension is fully
covered by three new tests in test_inbox.py (skip-from-queue,
cursor-advance-past-broken-fetch, URI-rewrite ordering).
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