feat: handle extension verification and retry-after on download

Classify verification-required and rate-limit errors from extensions, propagate Retry-After seconds through the download fallback, report session-grant success/failure, and add a completeGrant action fallback in the runtime.

lib/utils/extension_auth_launcher.dart

@@ -8,25 +8,43 @@ bool isExtensionVerificationRequired(Object error) {
   final message = error.toString().toLowerCase();
   return message.contains('verify_required') ||
       message.contains('verification_required') ||
+      message.contains('verification required') ||
       message.contains('needsverification') ||
-      message.contains('needs verification');
+      message.contains('needs verification') ||
+      message.contains('unauthorized') ||
+      message.contains('precondition required') ||
+      _containsHttpStatusCode(message, '401') ||
+      _containsHttpStatusCode(message, '428');
 }
 
-Future<void> openPendingExtensionVerification(String extensionId) async {
+bool _containsHttpStatusCode(String message, String code) {
+  return message.contains('http $code') ||
+      message.contains('http status $code') ||
+      message.contains('status $code') ||
+      message.contains('$code for ') ||
+      message.contains('$code:') ||
+      message.contains('$code;');
+}
+
+Future<bool> openPendingExtensionVerification(String extensionId) async {
   final normalizedExtensionId = extensionId.trim();
-  if (normalizedExtensionId.isEmpty) return;
+  if (normalizedExtensionId.isEmpty) return false;
 
   try {
     final pending = await PlatformBridge.getExtensionPendingAuth(
       normalizedExtensionId,
     );
     final authUrl = pending?['auth_url']?.toString().trim() ?? '';
-    if (authUrl.isEmpty) return;
+    if (authUrl.isEmpty) return false;
 
     final uri = Uri.tryParse(authUrl);
-    if (uri == null) return;
+    if (uri == null) return false;
+
+    var launched = await launchUrl(uri, mode: LaunchMode.inAppBrowserView);
+    if (!launched) {
+      launched = await launchUrl(uri, mode: LaunchMode.externalApplication);
+    }
 
-    final launched = await launchUrl(uri, mode: LaunchMode.externalApplication);
     if (launched) {
       _log.i('Opened verification challenge for $normalizedExtensionId');
     } else {
@@ -34,9 +52,11 @@ Future<void> openPendingExtensionVerification(String extensionId) async {
         'Could not open verification challenge for $normalizedExtensionId',
       );
     }
+    return launched;
   } catch (e) {
     _log.w(
       'Failed to open verification challenge for $normalizedExtensionId: $e',
     );
+    return false;
   }
 }