feat(usernames): update unique constraint to composite with type and provider

- Remove single unique constraint on username
  - Add composite unique constraint on username, username_type, and provider_type
  - Update Filament username form validation with custom unique rule
  - Add descriptive validation message for duplicate usernames

app/Filament/Resources/Usernames/Schemas/UsernameForm.php

@@ -9,7 +9,9 @@ use Filament\Forms\Components\Select;
 use Filament\Forms\Components\TextInput;
 use Filament\Forms\Components\ToggleButtons;
 use Filament\Infolists\Components\TextEntry;
+use Filament\Schemas\Components\Utilities\Get;
 use Filament\Schemas\Schema;
+use Illuminate\Validation\Rules\Unique;
 
 class UsernameForm
 {
@@ -21,7 +23,19 @@ class UsernameForm
                     ->columnSpan(1)
                     ->alphaDash()
                     ->helperText('Email: myusername@gmail.com | Username: myusername')
-                    ->required(),
+                    ->required()
+                    ->unique(
+                        table: 'usernames',
+                        ignoreRecord: true,
+                        modifyRuleUsing: function (Unique $rule, Get $get) {
+                            return $rule
+                                ->where('username_type', $get('username_type'))
+                                ->where('provider_type', $get('provider_type'));
+                        },
+                    )
+                    ->validationMessages([
+                        'unique' => 'The username already exists for this type and provider.',
+                    ]),
                 TextInput::make('daily_mailbox_limit')
                     ->integer()
                     ->minValue(1)