Combine required and dependentRequired error messages

[Sam-61s]

When required and dependentRequired fail at the same instance location, they previously produced multiple duplicate messages. This change consolidates those failures into a single required-message, combining and deduplicating missing properties while preserving schema locations.

Adds test coverage for the original issue scenario and keeps existing dependentRequired and dependency behavior unchanged.

Fixes #134

[jdesrosiers]

Try again. You should end up with one error handler that handles required, dependentRequired, and the array form of dependencies. There doesn't need to be a separate error handler for each keyword. They're defined separately specifically for that reason.

[Sam-61s]

Thanks for the clarification. I plan to unify required, dependentRequired, and the array-form of dependencies into required.js, while keeping schema-form dependencies handled separately through normal nested validation.

The unified handler will consolidate missing properties into a single required-message, deduplicate properties, and preserve schemaLocations. Since dependentRequired will be handled by the unified handler, I also plan to remove the separate dependentRequired.js handler.

Does that match your expectation?

[jdesrosiers]

Yep. That sounds right.