fix(v1/messages): reverse-translate OpenAI->Claude for all non-Claude providers

[mahdiwafy]

Summary

Reverse-translate OpenAI chat.completion responses back to Claude message format when the client sent a Claude-format request through a non-Claude provider (Gemini, Antigravity, etc.).

Changes

nonStreamingHandler.js (+72/-68 = net +4):

• Add openaiToClaudeNonStreaming() converter: maps OpenAI {choices, usage} → Claude {id, type, role, content, stop_reason, usage}
• Widen reverse-translation gate: check sourceFormat === FORMATS.CLAUDE && responseBody?.choices instead of targetFormat === FORMATS.OPENAI && sourceFormat === FORMATS.CLAUDE. Catches combo routing where selected provider is Gemini/Antigravity (not just OpenAI), since translateNonStreamingResponse always converts to OpenAI shape first
• Remove translateOpenAIToOpenAIResponses() dead code (4.8KB) — Responses API translation no longer happens in this path
• Pass comboName through to saveUsageStats

sseToJsonHandler.js (+29/-5 = net +24):

• Import openaiToClaudeNonStreaming from nonStreamingHandler
• Add Claude format reverse translation in SSE→JSON Chat Completions path (force-stream providers that aggregate SSE to OpenAI JSON)

Testing

Deployed at port 20126 and tested with Claude Code (Send Message) through a Gemini combo — returns proper Claude message format instead of raw OpenAI chat.completion JSON.

[Vanszs]

lot of conflict, please resolve it first 🗿

[mahdiwafy]

@Vanszs PR is clean and ready for review.

Merge check

• mergeable: ✅ MERGEABLE
• mergeStateStatus: ✅ CLEAN
• Base: main (up-to-date, 0 commits behind)
• Changed files: 2 files (+176/-126)
• Dry-run merge: Automatic merge went well — zero conflict

What this does

Reverse-translates OpenAI-format responses to Claude message format when the client sends Claude-format requests (/v1/messages). This allows Claude Code (and any Anthropic-format client) to work through VansRouter with any provider.

Both the standard non-streaming path and the SSE-to-JSON (force non-streaming) path are covered.

[Vanszs]

LGTM