Block API minor improvements

[Enmk]

New minor feature.

Block::Clear() convenience method that clears out all rows from all columns.
Block::Reserve() convenience method that increases capacity of all columns in the block.

Those methods are for the clients that re-use the blocks for multiple batched inserts.

Also ColumnLowCardinality::Reserve() now reserves sqrt(X) items for dictionary column instead of X -- this improves memory usage, since dictionary only holds unique keys of LC columns.

[Copilot]

Pull Request Overview

This PR introduces two new convenience methods in Block – Clear() to remove all rows and Reserve() to preallocate capacity – and adjusts the reservation strategy for LowCardinality columns to use a sqrt-based estimation. Additionally, it expands test coverage for ItemView serialization and block operations, while adding necessary declarations and minor improvements supporting these features.

• Added Block::Clear() and Block::Reserve() methods, with corresponding tests in ut/block_ut.cpp.
• Updated ColumnLowCardinality::Reserve() to reserve ceil(sqrt(new_cap)) for dictionary columns.
• Extended ItemView operator<< logic and integrated new includes and declarations.

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ut/utils_ut.cpp	Expanded tests for ItemView serialization with new includes.
ut/utils_comparison.h	Introduced minor maintainability comments in CompareRecursive logic.
ut/utils.h	Added ItemView operator<< declaration.
ut/utils.cpp	Updated DateTime conversion and ItemView operator<< implementation.
ut/block_ut.cpp	Added tests for the new Clear() and Reserve() Block methods.
clickhouse/columns/lowcardinality.cpp	Modified Reserve() to improve memory usage via sqrt estimation.
clickhouse/columns/itemview.h	Enabled get() support for UInt128 in ItemView.
clickhouse/block.h	Declared new Block API methods (Clear and Reserve).
clickhouse/block.cpp	Implemented newly introduced Block::Clear() and Block::Reserve().