randgen: disable generation of REGNAMESPACE type expressions

Casting an OID to REGNAMESPACE runs this SQL using the internal executor:
```
SELECT pg_namespace.oid, nspname FROM pg_catalog.pg_namespace WHERE oid = $1
```
When the `GenerateConstrainedScans` rule is disabled, this returns no
rows and so doesn't apply the cast. The same SQL, not run via an
internal executor, but also with `GenerateConstrainedScans` disabled,
behaves correctly. Disabling `GenerateConstrainedScans` all the time
prevents the cluster from starting.

The reason a full scan of pg_namespace doesn't find the OID is because it
only checks for schemas in the current database:
https://github.com/cockroachdb/cockroach/blob/7b341ffa678e4a22416e2274351fd56f415f5421/pkg/sql/virtual_schema.go#L602
https://github.com/cockroachdb/cockroach/blob/d10c3dd42c3dc40cad82792a30ae47fd2a663f43/pkg/sql/pg_catalog.go#L2099-L2106
https://github.com/cockroachdb/cockroach/blob/a7e9c4a68b81436d1f9382518d4267f74cbdac94/pkg/sql/information_schema.go#L2295-L2296

Whereas with a constrained scan, all descriptors are searched:
https://github.com/cockroachdb/cockroach/blob/af6a72a622ae05f3733b5db637403b3eaa9455f1/pkg/sql/catalog/descs/descriptor.go#L168-L175

The correct result is from the constrained scan, because we currently
allow cross-database references. Once #55791 is fixed, both results
should match.

The fix alternatives are:
1. disallow disabling of the `GenerateConstrainedScans` rule for internal SQL
2. turn off generation of REGNAMESPACE expressions in randgen to avoid hitting this problem in tests.

The 2nd fix alternative is implemented to avoid losing any of the
rule-disabling test coverage provided by the
`testing_optimizer_disable_rule_probability` setting.

Fixes #98322

Release note: None

pkg/sql/randgen/type.go

@@ -40,6 +40,10 @@ var (
 func init() {
 	for _, typ := range types.OidToType {
 		switch typ.Oid() {
+		case oid.T_regnamespace:
+			// Temporarily don't include this.
+			// TODO(msirek): Remove this exclusion once
+			// https://github.com/cockroachdb/cockroach/issues/55791 is fixed.
 		case oid.T_unknown, oid.T_anyelement:
 			// Don't include these.
 		case oid.T_anyarray, oid.T_oidvector, oid.T_int2vector: