fix: tighten and clean up null-rejection analysis in EliminateOuterJoin

The null-rejection analysis used by `EliminateOuterJoin` to decide
whether an outer join can be tightened had a soundness gap:

`IS TRUE` / `IS FALSE` / `IS NOT UNKNOWN` recursed into their operand
at any depth. These predicates return a definite FALSE on NULL input
(never NULL), so they're null-rejecting at the top of a WHERE clause
but not under a surrounding `NOT` (or any context that resets
`top_level`), where the FALSE inverts to TRUE and accepts NULL rows.
The existing `IS NOT NULL` arm already gated on `top_level`; the
three others now share the same gate.

While here, clean up several other rough edges:

* Defer to `Operator::returns_null_on_null()` instead of maintaining
  a hand-rolled list of null-propagating binary operators in
  `extract_non_nullable_columns`. The hand list previously only
  covered the six comparison operators; the upstream method is the
  single source of truth and also picks up arithmetic / bitwise /
  regex / SIMILAR TO / array-containment operators that were missed.
* Rename `extract_non_nullable_columns` to
  `extract_null_rejecting_columns`. The returned columns are columns
  the predicate rejects NULL on (a predicate property), not columns
  that are themselves non-nullable (a schema property). The two are
  distinct.
* Rewrite the OR / nested-AND merge logic from an O(n*m) double-loop
  with break into bucketed per-side lookups.
* Handle `Expr::Negative` alongside `Expr::Not` (both propagate NULL
  on their operand).
* Unify `IsNotNull` / `IsTrue` / `IsFalse` / `IsNotUnknown` into a
  single match arm now that they share the same `top_level` gate.
* Rewrite the function-level doc and inline comments.

Tests:

* 7 new unit tests covering the soundness fix (three
  `NOT(... IS TRUE/FALSE/NOT UNKNOWN)` regressions), the IS DISTINCT
  FROM / IS NOT DISTINCT FROM negative cases, and positive coverage
  for the arithmetic / unary-minus shapes that the operator-list
  extension enables.
* 1 new sqllogictest case asserting both the logical plan
  (`Left Join` preserved) and the row set (LEFT-padded NULL rows
  survive) for `WHERE NOT((t2.y > 150) IS TRUE)` — the row-set
  assertion is what catches a future regression that an explain-only
  test would let through under snapshot updates.

Author: neilconway