fix: return actual error message from token validation endpoint

- Return the specific error from validateProviderToken() instead of
  generic "Failed to validate token." message
- Update test to expect the actual error message

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/Http/Controllers/Api/CloudProviderTokensController.php

@@ -525,7 +525,7 @@ class CloudProviderTokensController extends Controller
 
         return response()->json([
             'valid' => $validation['valid'],
-            'message' => $validation['valid'] ? 'Token is valid.' : 'Failed to validate token.',
+            'message' => $validation['valid'] ? 'Token is valid.' : $validation['error'],
         ]);
     }
 }