Add test coverage for ActiveModel::Type::Binary - #58335

Summary

ActiveModel::Type::Binary's own test file covered cast and serialize only. Running line coverage over activemodel/test/cases/ showed active_model/type/binary.rb at 25/31 lines, with these unreached:

• type and binary?
• the Data branch of cast:

  def cast(value)
    if value.is_a?(Data)
      value.to_s   # <- unreached

• changed_in_place?
• Data#hex

Tests added

• test_type_and_binary_predicate — type is :binary and binary? is true.
• test_serialize_returns_binary_data — nil passes through; other values come back wrapped in Binary::Data.
• test_cast_binary_data_returns_the_underlying_string — round-trips a serialized value back through cast, asserting the Data branch returns the binary-encoded string.
• test_changed_in_place — true when the deserialized old value differs, false when it matches.
• test_data_hex_and_string_conversion — Data#hex on non-UTF-8 bytes, plus to_s/to_str and comparison against a raw string.

No behavior changes — tests only, so no CHANGELOG entry.

active_model/type/binary.rb goes from 25/31 to full line coverage. activemodel/test/cases/: 1199 runs, 5375 assertions, 0 failures, 0 errors. RuboCop clean.