fix(pull_requests): set default merge_method to 'merge' when not provided

When merge_method is not provided, default to 'merge' instead of
passing an empty string to the GitHub API, which could cause the
405 'Merge commits are not allowed' error on repos with squash/rebase
only settings.

Closes #2258

Author: Will-hxw

pkg/github/pullrequests.go

@@ -1307,6 +1307,9 @@ func MergePullRequest(t translations.TranslationHelperFunc) inventory.ServerTool
 			if err != nil {
 				return utils.NewToolResultError(err.Error()), nil, nil
 			}
+			if mergeMethod == "" {
+				mergeMethod = "merge"
+			}
 
 			options := &github.PullRequestOptions{
 				CommitTitle: commitTitle,