- Added `responseFormat` to request schema-constrained JSON output, implemented for Google, Google Vertex (`responseMimeType` plus `responseJsonSchema`), and OpenAI-compatible completions (`response_format: { type: "json_schema" }`). The completions path is enabled for endpoints known to support it and opt-in per model with `compat.supportsResponseFormatJsonSchema` for providers such as DeepSeek and z.ai that reject or ignore the field.
- Added `responseFormat` schema normalization that sends TypeBox literal unions as `enum`, because provider structured-output dialects do not accept `const`: Gemini ignores it and would accept any value, and OpenAI rejects the schema.