feat: Adds username validation in registration form

Introduces logic to clean and validate usernames by ensuring they remove the matrix domain prefix/suffix, are not empty, and contain only allowed characters. Enhances input validation for improved user error messaging.

src/synapse_registration/registration/forms.py

@@ -1,4 +1,5 @@
 from django import forms
+from django.conf import settings
 
 
 class UsernameForm(forms.Form):
@@ -9,6 +10,27 @@ class UsernameForm(forms.Form):
         ),
     )
 
+    def clean(self):
+        cleaned_data = super().clean()
+        username = cleaned_data.get("username")
+
+        if username.startswith("@") and username.endswith(f":{settings.MATRIX_DOMAIN}"):
+            username = username[1:-len(f":{settings.MATRIX_DOMAIN}")]
+
+        if not username:
+            self.add_error("username", "Username cannot be empty.")
+
+        if not all(
+            c in "abcdefghijklmnopqrstuvwxyz0123456789._=-" for c in username.lower()
+        ):
+            self.add_error(
+                "username",
+                "Username can only contain the characters a-z, 0-9, ., _, =, -, and /.",
+            )
+
+        cleaned_data["username"] = username
+        return cleaned_data
+
 
 class EmailForm(forms.Form):
     email = forms.EmailField(