Show instructor real name or nickname in group-class views, not the login

Add Auth\UserName::format(), which prefers a user's first + last name, then
their nickname, avoiding display_name (which can be the login/username).
Route the instructor name through it in both the front-end offerings response
(instructor_name) and the back-end group-class summary and details views.

Tests: composer test (513), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/Unit/Auth/UserNameTest.php

@@ -0,0 +1,47 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Auth;
+
+use Mockery;
+use Unsupervised\Schedular\Auth\UserName;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class UserNameTest extends TestCase
+{
+    private function user(string $first, string $last, string $nickname): \WP_User
+    {
+        $user = Mockery::mock(\WP_User::class);
+        $user->first_name = $first;
+        $user->last_name  = $last;
+        $user->nickname   = $nickname;
+
+        return $user;
+    }
+
+    public function testPrefersFirstAndLastName(): void
+    {
+        self::assertSame('Ada Lovelace', UserName::format($this->user('Ada', 'Lovelace', 'ada_login')));
+    }
+
+    public function testUsesFirstNameAloneWhenLastNameMissing(): void
+    {
+        self::assertSame('Ada', UserName::format($this->user('Ada', '', 'ada_login')));
+    }
+
+    public function testFallsBackToNicknameWhenNoRealName(): void
+    {
+        self::assertSame('Countess', UserName::format($this->user('', '', 'Countess')));
+    }
+
+    public function testFallsBackToIdWhenNothingSet(): void
+    {
+        self::assertSame('42', UserName::format($this->user('', '', ''), 42));
+    }
+
+    public function testReturnsFallbackIdWhenUserMissing(): void
+    {
+        self::assertSame('42', UserName::format(null, 42));
+        self::assertSame('', UserName::format(null));
+    }
+}