feat(studio): add structured-output prefill generation - #393
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Add a prefillStyle toggle to functionPrefill blocks. When set to 'structured', the block emits a strict JSON schema (response_format on the OpenAI wire, responseMimeType/responseSchema on native Gemini) instead of a synthetic tool_call. Gemini 3.8 rejects the legacy tool-call prefill with a missing thought_signature; structured output carries no such requirement, so the prefill keeps working. Default stays 'tool' — nothing changes unless a preset opts in.
Add a 'two-pass' prefill style. Instead of a synthetic tool call or a JSON response schema, the final generator runs twice: a quiet first pass produces the internal <thinking> block, then a second pass writes the visible reply seeded by that block. The reasoning is surfaced via AgentRunResult.reasoning. Survives Gemini 3.8's thought_signature requirement without tool_calls or a fragile schema.
The quiet thinking pass ran with isFinalResponse:false, which resolves the pre-gen controller model and its reasoning settings instead of the final model. Run it with isFinalResponse:true so both passes hit the final model + final reasoning config.
…plies The two-pass generator runs its final pass with stream=false, so the incoming <audit> reasoning block was never parsed out of the reply text and the model's second reasoning block leaked into content. Route the full non-streaming response through StreamAccumulator and preserve both reasoning blocks on the result.
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flutter analyze --no-fatal-infos --no-fatal-warnings(no errors; 9 pre-existing warnings/info)flutter test --reporter expanded: 3840 passed, 3 failed in 4m52stest/chat_input_bar_test.dartintroduced by upstream feat(chat): attach pasted images, up to four per message #389 (find.byType(Image)finds no thumbnail)