feat: support variant cast to timestamp_tz

[KKould]

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Summary

add variant functions for timestamp_tz

• fixes: Feature: support variant cast to timestamp_tz #19034

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[TCeason]

Please add Sqllogic test

[KKould]

@codex review

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[Copilot]

Pull request overview

This PR adds support for casting variant types to timestamp_tz (timestamp with timezone), addressing issue #19034. The implementation follows the same patterns used for other temporal types like timestamp, date, and interval.

Key changes:

• Adds to_timestamp_tz, try_to_timestamp_tz, as_timestamp_tz, and is_timestamp_tz functions for variant casting
• Updates type checking and evaluation logic to handle nullable timestamp_tz types
• Includes comprehensive unit and SQL logic tests

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/query/expression/src/type_check.rs	Fixed cast function selection to handle nullable timestamp_tz types by using remove_nullable()
src/query/expression/src/evaluator.rs	Added TimestampTz to the lists of supported cast target types from variant
src/query/functions/src/scalars/variant.rs	Implemented four new variant functions: is_timestamp_tz, as_timestamp_tz, to_timestamp_tz, and try_to_timestamp_tz, plus helper function cast_to_timestamp_tz
src/query/functions/tests/it/scalars/variant.rs	Added unit tests for all new timestamp_tz variant functions
src/query/functions/tests/it/scalars/testdata/variant.txt	Generated test output data for the new unit tests
tests/sqllogictests/suites/query/functions/02_0056_function_semi_structureds_as.test	Added SQL logic tests for timestamp_tz variant casting and type checking
tests/sqllogictests/suites/query/functions/02_0002_function_cast.test	Added SQL logic tests for timestamp_tz casting from JSON including null handling and error cases

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