[codex] Support timezone-qualified RowBinary datetimes

[ruslandoga]

Summary

• Preserve timezone-qualified DateTime and DateTime64 RowBinary encoding types instead of rejecting non-UTC annotations.
• Interpret NaiveDateTime values in the ClickHouse type timezone while continuing to encode DateTime values by instant.
• Add focused regression coverage and an Unreleased changelog entry.

Fixes #353.

Checks

• MIX_ENV=test mix format --check-formatted
• mix test test/ch/row_binary_test.exs
• Full suite delegated to isolated CI because concurrent local workspaces share the ClickHouse test database.