SQL/JSON: Fix coercion of constructor outputs to types with typmod

SQL/JSON: Fix coercion of constructor outputs to types with typmod

Ensure SQL/JSON constructor functions that allow specifying the
target type using the RETURNING clause perform implicit cast to
that type.  This ensures that output values that exceed the specified
length produce an error rather than being  silently truncated. This
behavior conforms to the SQL standard.

Reported-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/202405271326.5a5rprki64aw%40alvherre.pgsql