ActiveRecord::RecordInvalid: Validation failed: Exercise must exist (ActiveRecord::RecordInvalid)

## Error

```
Validation failed: Exercise must exist (ActiveRecord::RecordInvalid)
```

## Stack trace

```
active_record/validations.rb:84 in raise_validation_error
active_record/validations.rb:55 in save!
active_record/transactions.rb:313 in block in ActiveRecord::Transactions#save!
active_record/transactions.rb:365 in block in ActiveRecord::Transactions#with_transaction_returning_status
active_record/connection_adapters/abstract/database_statements.rb:342 in transaction
active_record/transactions.rb:361 in with_transaction_returning_status
active_record/transactions.rb:313 in save!
active_record/suppressor.rb:56 in save!
active_record/persistence.rb:906 in block in ActiveRecord::Persistence#update!
active_record/transactions.rb:365 in block in ActiveRecord::Transactions#with_transaction_returning_status
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [ActiveRecord::RecordInvalid: Validation failed: Exercise must exist (ActiveRecord::RecordInvalid)](https://thalamus-ai.sentry.io/issues/145735340/)
- **Culprit**: API::SolutionsController#sync
- **First seen:** Sep 8, 2026

Start at API::SolutionsController#sync and trace the save! call through the listed ActiveRecord files, using the Sentry event to reproduce the failure. Determine why the Exercise association is absent and add a regression check; done means the sync no longer raises this validation error.

Written by the indexing model from the issue text.