[FEATURE]: Support Responses API for deepseek-v4-flash on opencode-go

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

DeepSeek's formal deepseek-v4-flash-0731 checkpoint (released Jul 31, 2026) natively supports the OpenAI Responses API (https://api.deepseek.com → /v1/responses). However, opencode-go only serves deepseek-v4-flash via /v1/chat/completions:

• POST https://opencode.ai/zen/go/v1/responses with deepseek-v4-flash → HTTP 400: Error from provider (Console Go): Upstream request failed
• The same endpoint works fine for GPT-5.x models (e.g. gpt-5.6-luna)
• The same request succeeds against DeepSeek's official Responses API

This blocks Codex-style clients (which speak the Responses protocol) from using DeepSeek V4 Flash through opencode-go.

Proposed behavior:

1. Switch deepseek-v4-flash upstream to the 0731 checkpoint that supports Responses API natively.
2. Forward /v1/responses requests for deepseek-v4-flash to the upstream Responses endpoint (like GPT-5.x models).
3. Keep /v1/chat/completions working for backwards compatibility.

Additional context:

• DeepSeek official docs: https://api-docs.deepseek.com/zh-cn/guides/responses_api
• Related: #39823 (is the 0731 checkpoint live?), #39827 (Go gateway outage today)