Restore contsel/contjoinsel for containment & key-existence operators (#2356)

The containment (`@>`, `<@`, `@>>`, `<<@`) and key-existence (`?`, `?|`,
`?&`) operators on `agtype` were bound to `matchingsel`/`matchingjoinsel`
on the PG14+ source tree. `matchingsel` is built for pattern operators
(LIKE/regex) and during planning invokes the operator's underlying
function (`agtype_contains`) once per `pg_statistic` MCV. With realistic
statistics targets that produces a planner-time regression that
dominates simple OLTP-style point queries.

Restore the lighter `contsel`/`contjoinsel` helpers used by PostgreSQL
core's jsonb operators (`@>`, `<@`, `?` on jsonb), which matches
upstream's long-standing precedent for the same operator family.

Changes:

* `sql/agtype_operators.sql`, `sql/agtype_exists.sql`: 10 operators
  flipped from `matchingsel`/`matchingjoinsel` to
  `contsel`/`contjoinsel`.
* `age--1.7.0--y.y.y.sql`: appended `ALTER OPERATOR ... SET (RESTRICT,
  JOIN)` for all 10 operators so existing installs flip on
  `ALTER EXTENSION age UPDATE`.
* `regress/sql/containment_selectivity.sql` (+ `expected/.out`): pin
  the bindings via `pg_operator`, plus a "no leaked matchingsel"
  aggregate guard and functional smoke for all 10 operators. The
  guard catches future regressions if a new operator is added without
  the right selectivity helper.
* `regress/expected/cypher_match.out`, `regress/expected/cypher_vle.out`:
  refresh expected to reflect new (and better) plan shapes that the
  lower-selectivity helper produces — `test_enable_containment` now
  picks Nested Loop + Index Only Scans over a Seq Scan/Hash Join, and
  two `MATCH p=...` and `show_list_use_vle` queries flip row order
  (queries had no `ORDER BY`; result set is unchanged, only ordering).
* `Makefile`: register `containment_selectivity` in `REGRESS`.

Validation:

* Build: clean, `-Werror`.
* Regression: 36/37 tests pass under `EXTRA_TESTS="pgvector
  fuzzystrmatch pg_trgm"`. Only `age_upgrade` fails — pre-existing on
  master at 774e781 (verified by `git stash && installcheck`).
* Reporter's exact methodology (LDBC-SNB-style snb_graph + pgbench on
  `bench_message_content`) reproduces the regression and the fix:

  | Metric                     | matchingsel | contsel | Delta |
  |----------------------------|-------------|---------|-------|
  | EXPLAIN planning time (ms) | 1.42        | 0.97    | -32%  |
  | EXPLAIN execution time (ms)| 0.34        | 0.31    | ~equal|
  | pgbench TPS (8c x 30s)     | 5247        | 7378    | +40.6%|

  Run with `default_statistics_target = 1000` to populate MCV lists,
  matching the reporter's analyzed-graph conditions.

* Upgrade path: validated end-to-end during the benchmark — operator
  bindings were flipped from `matchingsel` -> `contsel` via the same
  `ALTER OPERATOR` statements the upgrade SQL ships, while operators
  remained functional throughout.

Driver workflows (python/go/node/jdbc) intentionally not run: this PR
only adjusts pg_operator selectivity metadata. There is no C, type, or
protocol change that drivers could observe.

Closes #2356.

Author: crprashant

Makefile

@@ -183,7 +183,8 @@ REGRESS = scan \
           direct_field_access \
           security \
           reserved_keyword_alias \
-          agtype_jsonb_cast
+          agtype_jsonb_cast \\
+          containment_selectivity
 
 ifneq ($(EXTRA_TESTS),)
   REGRESS += $(EXTRA_TESTS)