fix(auth): improve error message formatting

- Removed toString() from exception messages
- Fixed const exception usage

Author: fulleni

lib/src/services/auth_service.dart

@@ -282,7 +282,7 @@ class AuthService {
         'Error during initiateLinkEmailProcess for user ${anonymousUser.id}, email $emailToLink: $e',
       );
       throw OperationFailedException(
-        'Failed to initiate email linking process: ${e.toString()}',
+        'Failed to initiate email linking process: $e',
       );
     }
   }
@@ -369,7 +369,7 @@ class AuthService {
         'Error during completeLinkEmailProcess for user ${anonymousUser.id}: $e',
       );
       throw OperationFailedException(
-        'Failed to complete email linking process: ${e.toString()}',
+        'Failed to complete email linking process: $e',
       );
     }
   }

lib/src/services/verification_code_storage_service.dart

@@ -220,7 +220,7 @@ class InMemoryVerificationCodeStorageService
               userId,
     );
     if (isEmailPendingForOther) {
-      throw ConflictException(
+      throw const ConflictException(
         'Email is already pending verification for another account linking process.',
       );
     }

routes/api/v1/auth/link-email.dart

@@ -77,7 +77,7 @@ Future<Response> onRequest(RequestContext context) async {
       'Unexpected error in /link-email handler for user ${authenticatedUser.id}: $e',
     );
     throw OperationFailedException(
-      'An unexpected error occurred while initiating email linking: ${e.toString()}',
+      'An unexpected error occurred while initiating email linking: $e',
     );
   }
 }

routes/api/v1/auth/verify-link-email.dart

@@ -105,7 +105,7 @@ Future<Response> onRequest(RequestContext context) async {
       'Unexpected error in /verify-link-email handler for user ${authenticatedUser.id}: $e',
     );
     throw OperationFailedException(
-      'An unexpected error occurred while verifying email link: ${e.toString()}',
+      'An unexpected error occurred while verifying email link: $e',
     );
   }
 }