Fix: Username validation regex missing anchors

Description:

The current username validation regex /[0-9a-zA-Z-_.]+/ in EmbeddedChatApi.ts lacks start (^) and end ($) anchors. This causes the validation to pass for any string that contains at least one valid character, even if the rest of the string is invalid.

Steps to reproduce:

1. Call updateUserUsername(userid, "@#$a@#$").
2. Observe that usernameRegExp.test(newUserName) returns true because it matches the character "a".
3. The method proceeds to attempt an update with an invalid username format.

Expected behavior:

The validation should fail if the string contains any characters outside the allowed set [0-9a-zA-Z-_.].

Actual behavior:

The validation passes if at least one character in the string matches the regex.

Start in EmbeddedChatApi.ts and inspect the usernameRegExp check used by updateUserUsername. Reproduce the issue with "@#$a@#$" and verify that usernames containing characters outside the allowed set fail validation while valid usernames continue to pass.