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/openai_gateway_service.go

@@ -888,6 +888,22 @@ func isOpenAIInstructionsRequiredError(upstreamStatusCode int, upstreamMsg strin
 	return false
 }
 
+// ExtractSessionID extracts the raw session ID from headers or body without hashing.
+// Used by ForwardAsAnthropic to pass as prompt_cache_key for upstream cache.
+func (s *OpenAIGatewayService) ExtractSessionID(c *gin.Context, body []byte) string {
+	if c == nil {
+		return ""
+	}
+	sessionID := strings.TrimSpace(c.GetHeader("session_id"))
+	if sessionID == "" {
+		sessionID = strings.TrimSpace(c.GetHeader("conversation_id"))
+	}
+	if sessionID == "" && len(body) > 0 {
+		sessionID = strings.TrimSpace(gjson.GetBytes(body, "prompt_cache_key").String())
+	}
+	return sessionID
+}
+
 // GenerateSessionHash generates a sticky-session hash for OpenAI requests.
 //
 // Priority: