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_lapi_test.go

@@ -12,7 +12,6 @@ import (
 )
 
 func TestGetLAPIDecisions_FallbackToCscli(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 	router := gin.New()
 
 	// Create handler with mock executor
@@ -40,7 +39,6 @@ func TestGetLAPIDecisions_FallbackToCscli(t *testing.T) {
 }
 
 func TestGetLAPIDecisions_EmptyResponse(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 	router := gin.New()
 
 	// Create handler with mock executor that returns empty array
@@ -67,7 +65,6 @@ func TestGetLAPIDecisions_EmptyResponse(t *testing.T) {
 }
 
 func TestCheckLAPIHealth_Handler(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 	router := gin.New()
 
 	handler := &CrowdsecHandler{