2.11.x breaks structured-output requests routed via the opencode provider (400 "This response_format type is unavailable now") #1338

Environment:

• opencodex 2.11.0 and 2.11.1 (npm, Windows), downgrade to 2.10.2 resolves it
• Provider: opencode-free (opencode.ai gateway)
• Model: deepseek-v4-flash:free
• Client: OpenAI Codex desktop app (approval-reviewer escalations request structured output)

Regression:
opencodex 2.11.0 began forwarding response_format to upstream in src/adapters/openai-chat.ts:
when textFormat.type === "json_schema", it sets body.response_format = { type: "json_schema", json_schema: {...} }.
The opencode-free gateway rejects this with:

Provider error 400: {"error":{"param":null,"type":"invalid_request_error","code":"invalid_request_error","message":"Error from provider (Console): Upstream request failed: [invalid_request_error] This response_format type is unavailable now"}}

Impact:
Every request that asks for structured output fails - e.g., Codex's automatic approval reviewer always uses json_schema, so all escalated tool calls fail with this 400 under deepseek. Normal (non-structured) requests succeed. 2.10.2 does not forward response_format and works fine.

Expected:
Either stop sending response_format to providers that don't support it (e.g., opencode), map json_schema to a supported format for that provider, or document the limitation.