Encoding::CompatibilityError: incompatible character encodings: UTF-16LE and UTF-8 (Encoding::CompatibilityError)

## Error

```
incompatible character encodings: UTF-16LE and UTF-8 (Encoding::CompatibilityError)
```

## Stack trace

```
rack/query_parser.rb:132 in index
rack/query_parser.rb:132 in _normalize_params
rack/query_parser.rb:121 in normalize_params
rack/request.rb:688 in block in Rack::Request::Helpers#expand_param_pairs
rack/request.rb:687 in each
rack/request.rb:687 in expand_param_pairs
rack/request.rb:529 in POST
rack/method_override.rb:49 in method_override_param
rack/method_override.rb:33 in method_override
rack/method_override.rb:21 in call
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [Encoding::CompatibilityError: incompatible character encodings: UTF-16LE and UTF-8 (Encoding::CompatibilityError)](https://thalamus-ai.sentry.io/issues/120208924/)
- **Culprit**: String in index
- **First seen:** May 15, 2026