[instructor / Gemini] structured-output reask always fails: request ends on a model turn

## Observed symptom Gemini structured-output reask always fails: request ends on a model turn **Affected:** `instructor/v2/providers/genai/handlers.py::reask_genai_structured_outputs`, on `Mode.GENAI_STRUCTURED_OUTPUTS` (alias of `Mode.JSON` for `Provider.GENAI`). **Versions:** 1.16.0 (reproduced), 1.17.0 (function is byte-identical at the tag). **SDK:** google-genai 2.20.0. **Model:** gemini-3.7-flash (any Gemini model behaves the same). After a validation failure, `reask_genai_structured_outputs` appends the correction to `kwargs["contents"]` as a single `types.ModelContent`: ## Literal error text ``` 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': 'Requests ending with a model turn are not supported.', 'status': 'INVALID_ARGUMENT'}} ``` ## Environment GitHub issue 567-labs/instructor #2631; state=open ## Trigger / context Public GitHub report in 567-labs/instructor (FRESH_7D). ## Source evidence - https://github.com/567-labs/instructor/issues/2631 (2026-10-01T16:19:03Z) — evidence_class: PRIMARY_ISSUE_REPORT — GitHub issue #2631 (open) ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:40:08.726088Z - scout_id: C2934609308 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.