docs(astrolabe): add config docs and unit tests for internal URL

Address PR #487 reviewer feedback:

- Add documentation for `astrolabe_internal_url` config option
- Add unit tests for `IdpTokenRefresher::getNextcloudBaseUrl()`
- Fix CI workflow paths (astroglobe -> astrolabe)
- Add PHPUnit job to CI workflow for PHP 8.1, 8.2, 8.3
- Remove obsolete ApiTest that tested non-existent method

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

third_party/astrolabe/tests/unit/bootstrap.php

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Bootstrap for unit tests.
+ *
+ * Unit tests use mocked dependencies and don't require a full Nextcloud
+ * environment. This bootstrap only loads the composer autoloader which
+ * includes the OCP interface definitions needed for mocking.
+ */
+
+require_once __DIR__ . '/../../vendor/autoload.php';