[Feature]: Align z-order column parsing with ColumnName semantics

### Is your feature request related to a problem? _No response_ ### Describe the solution you'd like Z-Order currently uses naive dot-splitting for column paths (added in #2007 to support nested struct fields). This leaves an ambiguity: a column whose name literally contains a dot (e.g. `user.info`) cannot be distinguished from a nested path (`meta.field_a`). The kernel already ships a parser for this, `delta_kernel::expressions::ColumnName` supports backtick escaping (`` `user.info` `` for a literal name, `meta.field_a` for a nested path), and delta-rs already depends on it. keep the public API as `Vec<String>` for compatibility, but parse each Z-Order input string into a structured `ColumnName` up front and carry that path through validation (`validate_zorder_column`) and expression construction (`read_zorder`). This gets us both nested paths and literal dotted names without any optimize-specific escaping rules. follow-up to #2007 ### Describe alternatives you've considered _No response_ ### Priority None ### Additional context _No response_ ### Contribution - [x] I'm willing to submit a pull request for this feature - [ ] I can help with testing this feature - [ ] I can help with documentation for this feature