YQL-21315: Implement block variant comp node

2026-07-14T09:17:28Z

#### Implement Block Variant Computational Node ✎ - Introduces a new computational node for handling variant types in block operations, supporting both tuple and struct variants - Adds type annotation and runtime support for the new variant node with proper type checking and validation - Implements block-level variant operations that can select specific alternatives from variant types based on index or member name - Includes comprehensive unit tests and integration tests covering both tuple and struct variant scenarios - Updates runtime version to 81 and integrates the new node into the compilation and execution pipeline - Provides SQL syntax support for variant operations through the `Variant` function with appropriate type handling Autodescription by Yandex Code Assistant commit_hash:a07c1a365815a271cdd34958369156fd542fd5cc

ydb/yql/essentials/tests/sql/suites/blocks/variant_tuple.yql, branch main

YQL-21315: Implement block variant comp node