Allow a retention time zone on native timestamp columns

[mkuchenbecker]

Stacked on #732 (mkuchenbecker/retention-timezone-support); review that PR first.

What this changes

Native timestamp retention columns hold a wall clock in an arbitrary producer zone, not UTC. This lets the retention time zone declare that zone, the same way it already declares the zone of a string label. Before this change, a zone on a time-partitioned (native timestamp) table was rejected.

• The validator rejects a zone only when a string column's pattern already encodes a zone. It accepts a zone on a native timestamp column and on a zone-free string pattern.
• No delete-SQL change: the app builds now in the declared zone, and the native date_trunc predicate already reads that zoned wall clock, so the cutoff is evaluated in the column's zone.

Known limitation

With backup enabled, the Iceberg backup filter compares an absolute instant while the native delete compares a wall clock, so for a native column with a non-UTC zone the two can differ by the offset. Backup is a secondary path and metadata-only is not a hard requirement, so this is left as a follow-up rather than reintroducing a computed cutoff.

Tests

• Validator tests accept a zone on a native table and still reject a zone-bearing pattern.
• A SparkJobUtil test pins that the native statement interpolates the zoned wall clock.
• The no-zone regression suite is unchanged, so the UTC contract is unaffected.