fix: route GPT 5.6 Luna to Responses API + tool call flush + reasoning (#93)

[ltmoerdani]

Summary

Fix GPT 5.6 Luna on OpenCode Go — model was unusable in agent mode (tool calling) and reasoning/thinking was not working.

Root Causes Found

#	Root Cause	Fix
1	Routing wrong — GPT 5.6 Luna sent to chat-completions, but OpenCode Go docs require Responses API ()	Route all GPT models to Responses API
2	Tool calls not flushed — gateway sends finish_reason: null on final chunk instead of "tool_calls"; extractor only flushed on exact match	Flush pending tool calls when finish_reason is null + calls exist
3	Reasoning payload missing — buildResponsesRequestBody() did not include thinkingPayload	Add thinkingPayload to Responses API body
4	Reasoning format wrong — sent { reasoning_effort } (chat-completions format) instead of { reasoning: { effort } } (Responses API format)	Fix to nested Responses API format
5	Model not registered — gpt-5.6-luna missing from metadata and fallback lists	Add to Go vendor metadata + fallbackModels

Changes

• src/routing.ts — Route ALL GPT models to Responses API (per OpenCode Go docs)
• src/streaming.ts — Flush tool calls when finish_reason is null; add diagnostic logging for empty responses
• src/thinking.ts — Add openai thinking family (off/low/medium/high/xhigh); fix reasoning payload to nested format
• src/extension.ts — Add gpt-5.6-luna to Go metadata + fallback; add responsesUrl to Go provider; add openai to thinking settings + picker; add thinkingPayload to buildResponsesRequestBody()
• src/metadata.ts — Add gpt-5.6-luna limits (1.05M context, 128K output) + snapshot
• docs/issues/41-20260803-gpt56-luna-routing-fix.md — Full investigation doc

Testing

• ✅ npm run compile passes
• ✅ Simple chat with gpt-5.6-luna works (textChars=238)
• ✅ Agent mode tool calls now flush correctly (verified via diagnostic SSE logs)
• ✅ Reasoning payload sent in correct Responses API format

Verification

Verified that OpenCode Go docs require gpt-5.6-luna on /v1/responses endpoint:

| GPT 5.6 Luna | gpt-5.6-luna | https://opencode.ai/zen/go/v1/responses | @ai-sdk/openai |

Fixes #93