fix(api): add anchors to username validation regex

[sudorishabh]

This change hardens the username validation logic in the @embeddedchat/api package. Usernames that were previously "partially" valid will now be rejected, triggering the automatic username suggestion fallback.

Fixes #1201

Changes

• Updated packages/api/src/EmbeddedChatApi.ts:
• Regex changed from /[0-9a-zA-Z-_.]+/ to /^[0-9a-zA-Z-_.]+$/.

Impact

• Security/Stability: Prevents invalid usernames from being sent to the Rocket.Chat server during the update process.
• User Experience: Users with invalid display names (which are used to generate the default username) will more reliably be assigned a suggested valid username instead of an error-prone one.

Action Items

• No manual migration steps are required for users of the library. The change is internal to the updateUserUsername logic.

PR Test Details

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-1201 after approval.