perf(push_down_filter): skip filter revalidation

[kumarUjjawal]

Which issue does this PR close?

• Part of perf: push_down_filter is pathologically slow for some plans #20002.

Rationale for this change

PushDownFilter rebuilds many Filter nodes on very large plans. Each rebuild used Filter::try_new, which re-ran predicate typevalidation. On the sql_planner_extended benchmark, that repeated workwas a big part of planning time.

This change adds an unchecked filter constructor for internal optimizer use and uses it inside PushDownFilter when the predicate is already a known-valid filter expression.

What changes are included in this PR?

• add Filter::new_unchecked for internal optimizer use
• keep alias stripping the same as Filter::try_new
• switch PushDownFilter to use the unchecked path when rebuilding
  filters
• add focused tests for alias stripping and checked vs unchecked
  behavior

Are these changes tested?

Yes

Are there any user-facing changes?

No