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/crowdsec_archive_validation_test.go

@@ -251,7 +251,6 @@ func TestConfigArchiveValidator_RequiredFiles(t *testing.T) {
 // TestImportConfig_Validation tests the enhanced ImportConfig handler with validation.
 func TestImportConfig_Validation(t *testing.T) {
 	t.Parallel()
-	gin.SetMode(gin.TestMode)
 
 	db := OpenTestDB(t)
 	tmpDir := t.TempDir()
@@ -320,7 +319,6 @@ func TestImportConfig_Validation(t *testing.T) {
 // TestImportConfig_Rollback tests backup restoration on validation failure.
 func TestImportConfig_Rollback(t *testing.T) {
 	t.Parallel()
-	gin.SetMode(gin.TestMode)
 
 	db := OpenTestDB(t)
 	tmpDir := t.TempDir()