Plan nested window functions for Snowflake

[osipovartem]

Summary

• plan nested window functions in dependency order for the Snowflake SQL dialect
• materialize each innermost window level before rebasing and planning its parent level
• preserve the original SELECT expression names after staged planning
• retain the existing nested-window planning error for Generic and PostgreSQL-compatible behavior

Why

Snowflake accepts expressions such as:

SELECT SUM(SUM(value) OVER ()) OVER ()

A single DataFusion Window node cannot execute nested window expressions. The Snowflake planner path now lowers them into consecutive Window nodes, while other dialects keep the existing validation behavior.

This also enables Rustice to execute Snowflake's documented pattern where CONDITIONAL_TRUE_EVENT consumes a LAG(...) OVER (...) expression.

Validation

• cargo +1.97.0 test -p datafusion-sql --lib --tests (92 unit + 585 integration tests)
• cargo +1.97.0 test -p datafusion --test core_integration snowflake_nested_window_functions_execute_in_stages
• cargo +1.97.0 clippy -p datafusion-sql -p datafusion --lib --tests -- -D warnings
• cargo +1.97.0 fmt --all -- --check