Fix internal error from CollateExpr in SQL/JSON DEFAULT expressions

Fix internal error from CollateExpr in SQL/JSON DEFAULT expressions

SQL/JSON functions such as JSON_VALUE could fail with "unrecognized
node type" errors when a DEFAULT clause contained an explicit COLLATE
expression. That happened because assign_collations_walker() could
invoke exprSetCollation() on a JsonBehavior expression whose DEFAULT
still contained a CollateExpr, which exprSetCollation() does not
handle.

For example:

  SELECT JSON_VALUE('{"a":1}', '$.c' RETURNING text
                    DEFAULT 'A' COLLATE "C" ON EMPTY);

Fix by validating in transformJsonBehavior() that the DEFAULT
expression's collation matches the enclosing JSON expression’s
collation. In exprSetCollation(), replace the recursive call on the
JsonBehavior expression with an assertion that its collation already
matches the target, since the parser now enforces that condition.

Reported-by: Jian He <jian.universality@gmail.com>
Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CACJufxHVwYYSyiVQ6o+PsRX6zQ7rAFinh_fv1kCfTsT1xG4Zeg@mail.gmail.com
Backpatch-through: 17