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/ops_retry.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/Wei-Shaw/sub2api/internal/domain"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/ctxkey"
 	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/gin-gonic/gin"
@@ -528,7 +529,7 @@ func (s *OpsService) selectAccountForRetry(ctx context.Context, reqType opsRetry
 func extractRetryModelAndStream(reqType opsRetryRequestType, errorLog *OpsErrorLogDetail, body []byte) (model string, stream bool, err error) {
 	switch reqType {
 	case opsRetryTypeMessages:
-		parsed, parseErr := ParseGatewayRequest(body)
+		parsed, parseErr := ParseGatewayRequest(body, domain.PlatformAnthropic)
 		if parseErr != nil {
 			return "", false, fmt.Errorf("failed to parse messages request body: %w", parseErr)
 		}
@@ -596,7 +597,7 @@ func (s *OpsService) executeWithAccount(ctx context.Context, reqType opsRetryReq
 			if s.gatewayService == nil {
 				return &opsRetryExecution{status: opsRetryStatusFailed, errorMessage: "gateway service not available"}
 			}
-			parsedReq, parseErr := ParseGatewayRequest(body)
+			parsedReq, parseErr := ParseGatewayRequest(body, domain.PlatformAnthropic)
 			if parseErr != nil {
 				return &opsRetryExecution{status: opsRetryStatusFailed, errorMessage: "failed to parse request body"}
 			}