fix: resolve P0/P1 code quality issues

- Add context parameter to shouldClearStickySession in tests
- Return 501 NotImplemented for TestCredentials
- Remove unused user variable in redeem_service
- Add comment for context.Background goroutine in promo_service
- Uncomment sora_client_handler tests (17 skipped tests)
- Add math/rand usage comment in request_transformer
- Fix ModelError.Error() to use fmt.Sprintf
- Add NotImplemented error type to errors package
- Optimize SSE defaultMaxLineSize from 500MB to 10MB

backend/internal/pkg/models/interface.go

@@ -219,7 +219,7 @@ type ModelError struct {
 
 func (e *ModelError) Error() string {
 	if len(e.Args) > 0 {
-		return e.Message + ": " + fmt.Sprint(e.Args...)
+		return fmt.Sprintf(e.Message, e.Args...)
 	}
 	return e.Message
 }