ArgumentError: invalid byte sequence in UTF-8 (ArgumentError)

## Error

```
invalid byte sequence in UTF-8 (ArgumentError)

BLANK_RE.match?(self)
^^^^
```

## Stack trace

```
active_support/core_ext/object/blank.rb:127 in match?
active_support/core_ext/object/blank.rb:127 in blank?
active_record/coders/json.rb:11 in ActiveRecord::Coders::JSON.load
active_record/type/serialized.rb:22 in deserialize
active_model/type/helpers/mutable.rb:8 in cast
active_model/attribute.rb:199 in type_cast
active_model/attribute.rb:43 in value
active_model/attribute.rb:162 in changed_from_assignment?
active_model/attribute.rb:67 in changed?
active_model/attribute_mutation_tracker.rb:79 in attribute_changed?
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [ArgumentError: invalid byte sequence in UTF-8 (ArgumentError)](https://thalamus-ai.sentry.io/issues/94794814/)
- **Culprit**: API::Solutions::SubmissionsController#create
- **First seen:** Feb 9, 2026