fix: route OpenAI-compatible gateways to the Chat Completions adapter

[JeffC0628]

What changed

DeepSeek (and any OpenAI-compatible endpoint that does not fully implement the Responses API tool loop) rejects the second /responses request with 400 No tool output found for tool call ... (request_id: ...) once the agent sends a tool result back, so every tool-using task fails with a RUN_ERROR.

This adds an opt-in OPENAI_CHAT_COMPLETIONS=true flag: when set, the openai provider is served by TanStack's openaiChatCompletions adapter (/v1/chat/completions) instead of the Responses-API openaiText adapter. The default keeps using the Responses API, so official OpenAI and existing gateway setups are untouched.

• apps/server/src/engine/tanstack-agent.ts: choose openaiChatCompletions when OPENAI_CHAT_COMPLETIONS === "true".
• .env.example: document the flag.
• tests/helpers/model.ts: the Responses fixture now pins its wire format against a developer .env flag; adds a Chat Completions wire-format fixture.
• tests/gateway-chat-completions.test.ts: regression tests for both wire formats.

Verification

• End-to-end repro against DeepSeek (OPENAI_BASE_URL=https://api.deepseek.com, MODEL=openai/deepseek-chat): a tool-using task previously failed with 400 No tool output found for tool call ... (request_id: ...); with the flag set it completes the full tool loop (tool call → result → final reply) over /v1/chat/completions.
• pnpm exec tsc --noEmit — pass.
• pnpm --dir apps/mobile typecheck — pass.
• pnpm build:server — pass.
• pnpm test — full suite passes (208 tests, 0 failures), including the new gateway regression tests.
• pnpm lint: on a Windows checkout biome reports repo-wide CRLF line-ending formatting (pre-existing, affects all files). The changed TS files pass biome after line-ending normalization; CI checks out LF (* text=auto in .gitattributes).

Integration limits

• Not tested against live Anthropic / Gemini / other providers — those adapter paths are untouched.
• Not run: build:web / build:ios / build:android — the change is server-side only.