RestClient::ServiceUnavailable: 503 Service Unavailable (RestClient::ServiceUnavailable)

## Error

```
503 Service Unavailable (RestClient::ServiceUnavailable)
```

## Stack trace

```
restclient/abstract_response.rb:249 in exception_with_response
restclient/abstract_response.rb:129 in return!
restclient/request.rb:836 in process_result
restclient/request.rb:743 in block in RestClient::Request#transmit
net/http.rb:1626 in start
restclient/request.rb:727 in transmit
restclient/request.rb:163 in execute
restclient/request.rb:63 in RestClient::Request.execute
restclient.rb:70 in RestClient.post
app/commands/user/github_solution_syncer/github_app.rb:24 in User::GithubSolutionSyncer::GithubApp.generate_installation_token!
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [RestClient::ServiceUnavailable: 503 Service Unavailable (RestClient::ServiceUnavailable)](https://thalamus-ai.sentry.io/issues/131839886/)
- **Culprit**: Settings::GithubSyncerController#callback
- **First seen:** Jul 2, 2026

Start at app/commands/user/github_solution_syncer/github_app.rb:24 and follow the RestClient call shown in the stack trace. Review the linked Sentry event and the Settings::GithubSyncerController#callback path to determine the expected behavior for a 503 response. Done means the reported failure is addressed and the callback behavior is covered by an appropriate verification.

Written by the indexing model from the issue text.