Update FormResponse error detail serialization as default behavior

[aduth]

🛠 Summary of changes

Updates FormResponse to remove the serialize_error_details_only, which is now already treated as the baseline behavior. Additionally, it inverts the assumed default usage of FormResponse from hash-based errors to ActiveModel::Errors-based errors when creating an instance without any errors, resulting in a few more instances where errors is omitted from the resulting serialized hash.

This continues from #11846, and should be the last piece of work in the primary migration away from errors to error_details for typical FormResponse usage with ActiveModel::Errors (not hash) form validation.

📜 Testing Plan

Verify build passes.