chore: Refactor test setup for Gin framework

- Removed redundant `gin.SetMode(gin.TestMode)` calls from individual test files.
- Introduced a centralized `TestMain` function in `testmain_test.go` to set the Gin mode for all tests.
- Ensured consistent test environment setup across various handler test files.

backend/internal/api/handlers/security_handler_fixed_test.go

@@ -13,7 +13,6 @@ import (
 )
 
 func TestSecurityHandler_GetStatus_Fixed(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 
 	tests := []struct {
 		name           string