User picker APIs not returning matches for middle part of display name

Issue Summary

It is common for users to enter the middle part of a name and expect user pickers to return results. This is the case when matching on email addresses, but not display names.

Steps to Reproduce

1. Create an app that calls the /rest/api/3/groupuserpicker endpoint.
2. Ensure a user exists with:
   1. Display name: "Foo Bar"
   2. email address: fbar@example.com
3. Have the app call the endpoint with ?query=fo and observe the results.
4. Have the app call the endpoint with ?query=ex and observe the results.
5. Have the app call the endpoint with ?query=oo and observe the results.

Expected Results

At steps 3, 4, and 5, the user is returned in the results.

Actual Results

At steps 3 and 4, the user is returned in the results, but at step 5, not the user is not returned in the results.

Workaround

None

Context

See https://community.developer.atlassian.com/t/how-to-query-users-by-a-string-matching-middle-part-of-the-displayname/44973.