Add unit tests for UpdateUserForm JSON unmarshalling

- Introduced `form_test.go` to validate the behavior of the `UpdateUserForm` struct's JSON unmarshalling, specifically for the `profile_image` field.
- Added tests to ensure omitted `profile_image` is not treated as an update, null values clear the image, and valid URLs are preserved.
- Enhanced the `UpdateUserForm` and `UpdateProfileForm` structs to include logic for distinguishing between omitted and explicitly null profile images during JSON unmarshalling.

This update improves the robustness of user profile updates by ensuring correct handling of profile image data in JSON requests, enhancing overall data integrity in the user management system.

internal/user/service.go

@@ -204,7 +204,7 @@ func (s *userService) Update(ctx context.Context, id string, form *UpdateUserFor
 		user.Phone = form.Phone
 	}
 
-	if form.ProfileImage != nil {
+	if form.isProfileImageSet() {
 		user.ProfileImage = form.ProfileImage
 	}
 
@@ -601,7 +601,7 @@ func (s *userService) UpdateProfile(ctx context.Context, userID string, form *Up
 		}
 		user.Phone = form.Phone
 	}
-	if form.ProfileImage != nil {
+	if form.isProfileImageSet() {
 		user.ProfileImage = form.ProfileImage
 	}