PR #3422 ('allow OpenAI usage-only empty choices chunks') reintroduced the
assistant-content injection '[OmniRoute] Upstream returned an empty response.
Please retry.' for empty `choices: []` chunks that carry no valid usage. Clients
(Goose/opencode) feed that text back as a turn and spin in a retry loop -- the
exact regression #3400 had fixed by dropping the chunk.
Restore the drop behavior for the no-usage case while preserving #3422's
standards-compliant forwarding of usage-only `include_usage` final chunks.
Realign the mislabeled stream-utils test (it asserted the injection) and add a
dedicated regression guard.
Reported-by: @mochizzan
Refs: #3502, #3388, #3400, #3422
c3c21fd56a