ActiveRecord::RecordInvalid: Validation failed: Comment markdown can't be blank (ActiveRecord::RecordInvalid)

## Error

```
Validation failed: Comment markdown can't be blank (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!
app/commands/mentor/request/create.rb:31 in block in Mentor::Request::Create#create_request!
active_record/connection_adapters/abstract/transaction.rb:535 in block in ActiveRecord::ConnectionAdapters::TransactionManager#within_new_transaction
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [ActiveRecord::RecordInvalid: Validation failed: Comment markdown can't be blank (ActiveRecord::RecordInvalid)](https://thalamus-ai.sentry.io/issues/145804550/)
- **Culprit**: API::Solutions::MentorRequestsController#create
- **First seen:** Sep 8, 2026

Start with app/commands/mentor/request/create.rb at line 31 and trace the API::Solutions::MentorRequestsController#create flow shown in the stack trace. Use the linked Sentry report to determine why a Comment with blank markdown reaches save!, then confirm the intended behavior with maintainers. Done means the reported RecordInvalid failure is resolved for the triggering flow and covered by an appropriate regression test.

Written by the indexing model from the issue text.