(`OBJECT_STORE_BASE_URL`) and a named protocol provider
(`OBJECT_STORE_DEFAULT_PROTOCOL=s3`), chat.agent session snapshot writes
landed in the named provider but reads fell through to the default — so
the recovery boot couldn't find the snapshot it had just written.
After a mid-stream cancel, the missing snapshot triggered a fallback
replay path that dropped the user's follow-up message, leaving the chat
stuck in `submitted` indefinitely.
Fix:
- New `/api/v1/sessions/:id/snapshot-url` route handles PUT + GET
symmetrically — both prefix unprefixed keys with
`OBJECT_STORE_DEFAULT_PROTOCOL` so they always round-trip through the
same store.
- `Session.chatSnapshotStoragePath` persists the resolved URI on first
write so future protocol changes don't strand existing snapshots.
Reads prefer the stored URI and fall back to the computed default for
pre-column sessions.
- SDK calls `createChatSnapshotUploadUrl` / `getChatSnapshotUrl`; the
generic v1/v2 packets endpoints are unchanged.
## Test plan
- [x] Configure local with two providers (R2 default + MinIO `s3` named)
and `OBJECT_STORE_DEFAULT_PROTOCOL=s3`.
- [x] Reproduce hang: send a message, cancel mid-stream, send another —
without the fix it hangs in `submitted`; with the fix it streams.
- [x] Snapshot lands in the `s3`-protocol bucket and
`Session.chatSnapshotStoragePath` is set after first write.
- [x] SDK unit tests pass; webapp typecheck passes.
Eric Allam
d343727021