Cannot add an author with a one character long name

Some people have names which are one character in length. A guest author with such a name cannot be assigned as the author of a post by typing their name.

Steps to reproduce:

- Visit "Add New Guest Author" page ( /wp-admin/post-new.php?post_type=guest-author )
- Enter "A" in "Display Name"
- Click "Add New Guest Author"
- Visit the new post page ( /wp-admin/post-new.php )
- Enter "A" in "Search for an author"

The list of search suggestions never appears.

As a workaround, you can use the guest author's ID (found in the URL when you're editing the guest author). That is assuming the guest author's ID is over 1 digit long.

Merely updating the call to `suggest()` in `js/co-authors-plus.js` to `suggest(..., { ..., minchars: 1 })` doesn't consistently work. Sometimes the dropdown appears after typing 1 character and sometimes it doesn't. I didn't do any further work on creating a PR for this bug.

Start by reproducing the one-character guest-author search from the WordPress admin steps in the issue, then inspect js/co-authors-plus.js and its suggest() call. Verify the behavior with a one-character display name and confirm that the matching suggestion appears reliably without relying on the guest-author ID.

Written by the indexing model from the issue text.