Cannot Login via MicrosoftAccount to RDP

### Describe the Bug

Cannot login into RDP session via `MicrosoftAccount\username@email.com`

`[invalid username @ crates/ironrdp-connector/src/credssp.rs:104] custom error: mixed username format`

This Login format works via Microsoft RDP application on desktop, but do not work via Pangolin RDP.

[Remote Desktop with Microsoft Account Sign-In](https://cmdrkeene.com/remote-desktop-with-microsoft-account-sign-in/)

### Environment

- OS Type & Version: Debian 13
- Pangolin Version: 1.22.2
- Edition (Community or Enterprise): Enterprise
- Newt Version: 1.16

### To Reproduce

Create Windows VM, sign into VM via Microsoft Account.
Enable the RDP, allow the user to use RDP to this machine
Create RDP resource, point to port 3389 and IP of the VM.
Try to login via Microsoft Account domain, emial of the microsoft account, and password of the account - you will get the error.

You can try to enter `MicrosoftAccount` into domain field, and `Email` of the account into Username field or enter `MicrosoftAccount\email` to username field. Both options ends with error `[invalid username @ crates/ironrdp-connector/src/credssp.rs:104] custom error: mixed username format`

### AI Disclosure

nope

### Expected Behavior

RDP should allow login via Microsoft account (probably the email of the account is the problem, but not sure)

Start at crates/ironrdp-connector/src/credssp.rs:104 and reproduce the login using the MicrosoftAccount domain and email formats described in the issue. Trace how those credentials reach the mixed username format error; done means a Microsoft Account can authenticate to the RDP session without that error.

Written by the indexing model from the issue text.