feat(openai): add /v1/messages endpoint and API compatibility layer

Add Anthropic Messages API support for OpenAI platform groups, enabling
clients using Claude-style /v1/messages format to access OpenAI accounts
through automatic protocol conversion.

- Add apicompat package with type definitions and bidirectional converters
  (Anthropic ↔ Chat, Chat ↔ Responses, Anthropic ↔ Responses)
- Implement /v1/messages endpoint for OpenAI gateway with streaming support
- Add model mapping UI for OpenAI OAuth accounts (whitelist + mapping modes)
- Support prompt caching fields and codex OAuth transforms
- Fix tool call ID conversion for Responses API (fc_ prefix)
- Ensure function_call_output has non-empty output field

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/gateway_service.go

@@ -5421,6 +5421,11 @@ func extractUpstreamErrorMessage(body []byte) string {
 		return m
 	}
 
+	// ChatGPT 内部 API 风格：{"detail":"..."}
+	if d := gjson.GetBytes(body, "detail").String(); strings.TrimSpace(d) != "" {
+		return d
+	}
+
 	// 兜底：尝试顶层 message
 	return gjson.GetBytes(body, "message").String()
 }