The workspace inbox poller filters
`GET /workspaces/:id/activity?type=a2a_receive` — writing rows with
`activity_type=chat_upload_receive` would be silently invisible to it.
Switch the poll-mode upload-staging handler to write
`activity_type=a2a_receive` with `method=chat_upload_receive` as the
discriminator. Same shape as A2A's `tasks/send` vs `message/send` method
split; the workspace-side handler (Phase 2) routes by `method`, not
activity_type.
Pinned with `TestPollUpload_ActivityRowDiscriminator` — sqlmock
WithArgs on positions 2 (activity_type) and 5 (method) so a refactor
that flips activity_type back to a custom value gets a red test
instead of a runtime "poller saw nothing" silent break.
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