fix: parse Gemini native request format in ParseGatewayRequest for correct session hash generation

ParseGatewayRequest only parsed Anthropic format (system/messages),
ignoring Gemini native format (systemInstruction/contents). This caused
GenerateSessionHash to produce identical hashes for all Gemini sessions.

Add protocol parameter to ParseGatewayRequest to branch between
Anthropic and Gemini parsing. Update GenerateSessionHash message
traversal to extract text from both formats.

backend/internal/service/gateway_service.go

@@ -510,9 +510,20 @@ func (s *GatewayService) GenerateSessionHash(parsed *ParsedRequest) string {
 	}
 	for _, msg := range parsed.Messages {
 		if m, ok := msg.(map[string]any); ok {
-			msgText := s.extractTextFromContent(m["content"])
-			if msgText != "" {
-				_, _ = combined.WriteString(msgText)
+			if content, exists := m["content"]; exists {
+				// Anthropic: messages[].content
+				if msgText := s.extractTextFromContent(content); msgText != "" {
+					_, _ = combined.WriteString(msgText)
+				}
+			} else if parts, ok := m["parts"].([]any); ok {
+				// Gemini: contents[].parts[].text
+				for _, part := range parts {
+					if partMap, ok := part.(map[string]any); ok {
+						if text, ok := partMap["text"].(string); ok {
+							_, _ = combined.WriteString(text)
+						}
+					}
+				}
 			}
 		}
 	}