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_cache_test.go

@@ -21,7 +21,6 @@ func (t *testCacheInvalidator) InvalidateCache() {
 }
 
 func TestSecurityHandler_ToggleSecurityModule_InvalidatesCache(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 	db := setupTestDB(t)
 	require.NoError(t, db.AutoMigrate(&models.Setting{}))