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/system_handler_test.go

@@ -44,7 +44,6 @@ func TestGetClientIPHeadersAndRemoteAddr(t *testing.T) {
 }
 
 func TestGetMyIPHandler(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 	r := gin.New()
 	handler := NewSystemHandler()
 	r.GET("/myip", handler.GetMyIP)