fix(gateway): 防止 OpenAI Codex 跨用户串流

根因：多个用户共享同一 OAuth 账号时，conversation_id/session_id 头
未做用户隔离，导致上游 chatgpt.com 将不同用户的请求关联到同一会话。

HTTP SSE 修复:
- 新增 isolateOpenAISessionID(apiKeyID, raw)，将 API Key ID 混入
  session 标识符（xxhash），确保不同 Key 的用户产生不同上游会话
- buildUpstreamRequest: OAuth 分支先 Del 客户端透传的 session 头，
  再用隔离值覆盖
- buildUpstreamRequestOpenAIPassthrough: 透传路径同样隔离
- ForwardAsAnthropic: Anthropic Messages 兼容路径同步修复
- buildOpenAIWSHeaders: WS 路径的 OAuth session 头同步隔离

backend/internal/service/openai_gateway_messages.go

@@ -107,10 +107,11 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 		return nil, fmt.Errorf("build upstream request: %w", err)
 	}
 
-	// Override session_id with a deterministic UUID derived from the sticky
-	// session key (buildUpstreamRequest may have set it to the raw value).
+	// Override session_id with a deterministic UUID derived from the isolated
+	// session key, ensuring different API keys produce different upstream sessions.
 	if promptCacheKey != "" {
-		upstreamReq.Header.Set("session_id", generateSessionUUID(promptCacheKey))
+		apiKeyID := getAPIKeyIDFromContext(c)
+		upstreamReq.Header.Set("session_id", generateSessionUUID(isolateOpenAISessionID(apiKeyID, promptCacheKey)))
 	}
 
 	// 7. Send request