Verify Rails 7+ encrypted attributes + serialized types work end-to-end (regression tests + docs)

[sncalvo]

Problem

Session#normalize_record calls record.attributes_for_database, which should handle Rails 7+ encrypted attributes and custom serialized types correctly — but there are no regression tests, and the behavior is not documented. A user finding out via a production data corruption is the worst case.

Proposal

• Add regression tests covering: encrypts :ssn, serialize :preferences, JSON, attribute :status, EnumType.new
• Assert that encrypted values land on the target encrypted, not in cleartext
• Assert round-trip equivalence
• Document the support matrix in README

Acceptance criteria

• Encrypted attribute test passes on Postgres, MySQL, SQLite
• Serialized attribute test passes
• README has a "Supported attribute types" section (or note under Usage) listing these cases explicitly
• Any discovered bugs filed as separate issues and fixed before closing this one