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_cache_verification_test.go

@@ -16,7 +16,6 @@ import (
 
 // TestListPresetsShowsCachedStatus verifies the /presets endpoint marks cached presets.
 func TestListPresetsShowsCachedStatus(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 
 	cacheDir := t.TempDir()
 	dataDir := t.TempDir()