[iceberg] Fix ClassCastException reading timestamp with local timezone columns

[ArnavBalyan]

Purpose

• Reading a timestamp with local timezone column from the Iceberg manifest throws ClassCastException. In IcebergConversions.toPaimonObject the two timestamp variants share a switch arm that casts the data type to TimestampType, but LocalZonedTimestampType is not a subtype of TimestampType.
• Split the arm so each case casts to its own type. Write side was already doing this.
• Fixes: [Bug] Fix ClassCastException reading TIMESTAMP_WITH_LOCAL_TIME_ZONE columns #7763

Tests

• UT

[ArnavBalyan]

cc @JingsongLi thanks!