fix(openai): avoid implicit image sticky sessions

backend/internal/service/openai_gateway_service.go

@@ -1125,6 +1125,35 @@ func (s *OpenAIGatewayService) ExtractSessionID(c *gin.Context, body []byte) str
 	return sessionID
 }
 
+func explicitOpenAISessionID(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
+}
+
+// GenerateExplicitSessionHash generates a sticky-session hash only from explicit
+// client session signals. It intentionally skips content-derived fallback and is
+// used by stateless endpoints such as /v1/images.
+func (s *OpenAIGatewayService) GenerateExplicitSessionHash(c *gin.Context, body []byte) string {
+	sessionID := explicitOpenAISessionID(c, body)
+	if sessionID == "" {
+		return ""
+	}
+
+	currentHash, legacyHash := deriveOpenAISessionHashes(sessionID)
+	attachOpenAILegacySessionHashToGin(c, legacyHash)
+	return currentHash
+}
+
 // GenerateSessionHash generates a sticky-session hash for OpenAI requests.
 //
 // Priority:
@@ -1137,13 +1166,7 @@ func (s *OpenAIGatewayService) GenerateSessionHash(c *gin.Context, body []byte)
 		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())
-	}
+	sessionID := explicitOpenAISessionID(c, body)
 	if sessionID == "" && len(body) > 0 {
 		sessionID = deriveOpenAIContentSessionSeed(body)
 	}