Extend physical CSE optimizer to handle AggregateExec

Scan aggregate function arguments for common subexpressions in
first-stage (Partial/Single) aggregates, insert an intermediate
ProjectionExec to pre-compute them, and rewrite the aggregate
expressions to reference the new columns. Also removes debug
logging added during development.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: andygrove