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_service.go

@@ -24,6 +24,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/pkg/openai"
 	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
 	"github.com/Wei-Shaw/sub2api/internal/util/urlvalidator"
+	"github.com/cespare/xxhash/v2"
 	"github.com/gin-gonic/gin"
 	"github.com/google/uuid"
 	"github.com/tidwall/gjson"
@@ -787,6 +788,20 @@ func getAPIKeyIDFromContext(c *gin.Context) int64 {
 	return apiKey.ID
 }
 
+// isolateOpenAISessionID 将 apiKeyID 混入 session 标识符，
+// 确保不同 API Key 的用户即使使用相同的原始 session_id/conversation_id，
+// 到达上游的标识符也不同，防止跨用户会话碰撞。
+func isolateOpenAISessionID(apiKeyID int64, raw string) string {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return ""
+	}
+	h := xxhash.New()
+	_, _ = fmt.Fprintf(h, "k%d:", apiKeyID)
+	_, _ = h.WriteString(raw)
+	return fmt.Sprintf("%016x", h.Sum64())
+}
+
 func logCodexCLIOnlyDetection(ctx context.Context, c *gin.Context, account *Account, apiKeyID int64, result CodexClientRestrictionDetectionResult, body []byte) {
 	if !result.Enabled {
 		return
@@ -2501,13 +2516,17 @@ func (s *OpenAIGatewayService) buildUpstreamRequestOpenAIPassthrough(
 		if chatgptAccountID := account.GetChatGPTAccountID(); chatgptAccountID != "" {
 			req.Header.Set("chatgpt-account-id", chatgptAccountID)
 		}
+		apiKeyID := getAPIKeyIDFromContext(c)
+		// 先保存客户端原始值，再做 compact 补充，避免后续统一隔离时读到已处理的值。
+		clientSessionID := strings.TrimSpace(req.Header.Get("session_id"))
+		clientConversationID := strings.TrimSpace(req.Header.Get("conversation_id"))
 		if isOpenAIResponsesCompactPath(c) {
 			req.Header.Set("accept", "application/json")
 			if req.Header.Get("version") == "" {
 				req.Header.Set("version", codexCLIVersion)
 			}
-			if req.Header.Get("session_id") == "" {
-				req.Header.Set("session_id", resolveOpenAICompactSessionID(c))
+			if clientSessionID == "" {
+				clientSessionID = resolveOpenAICompactSessionID(c)
 			}
 		} else if req.Header.Get("accept") == "" {
 			req.Header.Set("accept", "text/event-stream")
@@ -2518,13 +2537,18 @@ func (s *OpenAIGatewayService) buildUpstreamRequestOpenAIPassthrough(
 		if req.Header.Get("originator") == "" {
 			req.Header.Set("originator", "codex_cli_rs")
 		}
-		if promptCacheKey != "" {
-			if req.Header.Get("conversation_id") == "" {
-				req.Header.Set("conversation_id", promptCacheKey)
-			}
-			if req.Header.Get("session_id") == "" {
-				req.Header.Set("session_id", promptCacheKey)
-			}
+		// 用隔离后的 session 标识符覆盖客户端透传值，防止跨用户会话碰撞。
+		if clientSessionID == "" {
+			clientSessionID = promptCacheKey
+		}
+		if clientConversationID == "" {
+			clientConversationID = promptCacheKey
+		}
+		if clientSessionID != "" {
+			req.Header.Set("session_id", isolateOpenAISessionID(apiKeyID, clientSessionID))
+		}
+		if clientConversationID != "" {
+			req.Header.Set("conversation_id", isolateOpenAISessionID(apiKeyID, clientConversationID))
 		}
 	}
 
@@ -2887,22 +2911,27 @@ func (s *OpenAIGatewayService) buildUpstreamRequest(ctx context.Context, c *gin.
 		}
 	}
 	if account.Type == AccountTypeOAuth {
+		// 清除客户端透传的 session 头，后续用隔离后的值重新设置，防止跨用户会话碰撞。
+		req.Header.Del("conversation_id")
+		req.Header.Del("session_id")
+
 		req.Header.Set("OpenAI-Beta", "responses=experimental")
 		req.Header.Set("originator", resolveOpenAIUpstreamOriginator(c, isCodexCLI))
+		apiKeyID := getAPIKeyIDFromContext(c)
 		if isOpenAIResponsesCompactPath(c) {
 			req.Header.Set("accept", "application/json")
 			if req.Header.Get("version") == "" {
 				req.Header.Set("version", codexCLIVersion)
 			}
-			if req.Header.Get("session_id") == "" {
-				req.Header.Set("session_id", resolveOpenAICompactSessionID(c))
-			}
+			compactSession := resolveOpenAICompactSessionID(c)
+			req.Header.Set("session_id", isolateOpenAISessionID(apiKeyID, compactSession))
 		} else {
 			req.Header.Set("accept", "text/event-stream")
 		}
 		if promptCacheKey != "" {
-			req.Header.Set("conversation_id", promptCacheKey)
-			req.Header.Set("session_id", promptCacheKey)
+			isolated := isolateOpenAISessionID(apiKeyID, promptCacheKey)
+			req.Header.Set("conversation_id", isolated)
+			req.Header.Set("session_id", isolated)
 		}
 	}