two-character usernames don't appear in invite search

### Steps to reproduce

1. spaces > pick a space > view members > invite
2. search for new user "ab"

### Outcome

#### What did you expect?

see @ab:mymatrixspace.net

#### What happened instead?

no results

#### More details

- including the colon in the search term ("ab:") will find the user. This is not necessary for longer usernames.
- this does not happen in the member search once they are invited into the space
- this does not happen in the web client

### Your phone model

Pixel 8

### Operating system version

16

### Application version and app store

26.03.3

### Homeserver

synapse 1.149.1

### Will you send logs?

No

### Are you willing to provide a PR?

No