Logical Theme: Added OIDC_AUTH_PRE_REDIRECT event

Allows customization of the auth URL before the user is redirected
to that URL.
Related to #6014

Author: ssddanbrown

app/Access/Oidc/OidcService.php

@@ -49,6 +49,11 @@ public function login(): array
         $url = $provider->getAuthorizationUrl();
         session()->put('oidc_pkce_code', $provider->getPkceCode() ?? '');
 
+        $returnUrl = Theme::dispatch(ThemeEvents::OIDC_AUTH_PRE_REDIRECT, $url);
+        if (is_string($returnUrl)) {
+            $url = $returnUrl;
+        }
+
         return [
             'url'   => $url,
             'state' => $provider->getState(),

app/Theming/ThemeEvents.php

@@ -87,6 +87,17 @@ class ThemeEvents
      */
     const COMMONMARK_ENVIRONMENT_CONFIGURE = 'commonmark_environment_configure';
 
+    /**
+     * OIDC auth pre-redirect event.
+     * Runs just before BookStack redirects the user to the identity provider for authentication.
+     * Provides the redirect URL that will be used.
+     * If the listener returns a string value, that will be used as the redirect URL instead.
+     *
+     * @param string $redirectUrl
+     * @return string|null
+     */
+    const OIDC_AUTH_PRE_REDIRECT = 'oidc_auth_pre_redirect';
+
     /**
      * OIDC ID token pre-validate event.
      * Runs just before BookStack validates the user ID token data upon login.

tests/Auth/OidcTest.php

@@ -822,6 +822,34 @@ public function test_oidc_id_token_pre_validate_theme_event_with_return()
         ]);
     }
 
+    public function test_oidc_auth_pre_redirect_theme_event_with_return()
+    {
+        $args = [];
+        $callback = function (...$eventArgs) use (&$args) {
+            $args = $eventArgs;
+            return 'https://cats.example.com?beans=true';
+        };
+        Theme::listen(ThemeEvents::OIDC_AUTH_PRE_REDIRECT, $callback);
+
+        $resp = $this->post('/oidc/login');
+        $resp->assertRedirect('https://cats.example.com?beans=true');
+
+        $this->assertCount(1, $args);
+        $this->assertStringStartsWith('https://oidc.local/auth', $args[0]);
+    }
+
+    public function test_oidc_auth_pre_redirect_theme_event_with_no_return()
+    {
+        $callback = function ($redirectUrl) {
+            $redirectUrl = 'cat';
+        };
+        Theme::listen(ThemeEvents::OIDC_AUTH_PRE_REDIRECT, $callback);
+
+        $resp = $this->post('/oidc/login');
+        $redirect = $resp->headers->get('Location');
+        $this->assertStringStartsWith('https://oidc.local/auth?', $redirect);
+    }
+
     public function test_pkce_used_on_authorize_and_access()
     {
         // Start auth