Split PushDownFilter::rewrite into per-plan-node pushdown helpers #26305

Is your feature request related to a problem or challenge?

PushDownFilter::rewrite (push_down_filter.rs#L860-L1477) is a 463-line function with cyclomatic complexity 70 and cognitive complexity 110, and push_down_filter.rs has had 44 commits in the last 12 months.

It first simplifies and reorders the predicates (about L874-L917, including a subtle predicates_changed computation at about L895-L909), handles FALSE and fetch/skip early-outs, and then matches on the child plan. Simple arms (Sort, Repartition, Distinct) are fine, but several inline multi-step pushdown logic: SubqueryAlias column remapping (about L986-L1004), Unnest predicate classification (about L1018-L1084), Aggregate (about L1115), Window (about L1171), AsOfJoin (about L1261), TableScan (about L1315) and Extension (about L1401).

Every new plan type or pushdown tweak touches this one function, and the large arms can't be unit tested on their own.

Describe the solution you'd like

Keep rewrite as a dispatcher. Extract the preamble into simplify_filter_predicates(&mut Filter) -> Result<bool>, and move each non-trivial arm into a free function following the existing push_down_all_join / rewrite_projection pattern: push_down_subquery_alias, push_down_unnest, push_down_through_aggregate, push_down_through_window, push_down_through_asof_join, push_down_into_table_scan, push_down_through_extension. Optionally turn the file into a module directory (join, scan, simplify).

Describe alternatives you've considered

Leave it as is.

Additional context

Line references are pinned to commit 0d79252.