feat(maps): set propertyNames for string map keys and support oneOf

[davidkna]

This PR:

1. Implements the propertyNames field for map schemas (similar to feat: add enum map key #384), but with validation logic to only add it for string-typed keys. This avoids validator confusion (as in the test suite) with non-string key types.
2. Adds support for oneOf in addition to anyOf (added via Added better map support #452) when analyzing map key schemas. This enables proper handling of enums with documentation comments, which generate oneOf schemas instead of anyOf.