Reject nested window functions in the Snowflake dialect - #69

Summary

• remove dialect-specific staged planning for window functions nested inside other window functions
• restore DataFusion planning validation for the Snowflake dialect
• retain supported aggregate-inside-window planning
• add a Snowflake-dialect regression test for the rejection

Snowflake oracle

The previous behavior was based on an incorrect compatibility assumption. Snowflake rejects:

SELECT SUM(SUM(column1) OVER ()) OVER ()
FROM VALUES (1), (2), (3);

with error 002062 (42601): Window function ... may not be nested inside another window function.

Validation

• cargo +1.97.0 test -p datafusion-sql --lib --tests (92 unit + 586 integration passed)
• targeted nested-window and aggregate-inside-window tests passed
• cargo +1.97.0 clippy -p datafusion-sql --all-targets -- -D warnings
• cargo +1.97.0 fmt --check
• git diff --check