No validation that prevents creating a V2 table with `timestamp_ns/timestamptz_ns` types

Currently, there is no validation that prevents creating a V2 table with [V3-only types like `timestamp_ns/timestamptz_ns`](https://iceberg.apache.org/spec/#version-3-extended-types-and-capabilities).

Start by locating the Rust validation path used when creating V2 tables and inspect how timestamp_ns and timestamptz_ns are classified. The work is done when V2 table creation rejects these V3-only types while valid V2 types remain accepted, with coverage added for the validation behavior.

Written by the indexing model from the issue text.