feat: model PostgreSQL table-level NOT NULL constraints

[minleejae]

In POSTGRESQL mode, support PostgreSQL 18 table constraints such as ALTER TABLE t ADD CONSTRAINT nn NOT NULL id NOT VALID and CREATE TABLE t (id INT, CONSTRAINT nn NOT NULL id), including unnamed constraints and NO INHERIT.

Expose a NotNullConstraint with an editable target Column, optional constraint name, inheritance flag and existing ConstraintAttributes validation flag. Add Index.Kind.NOT_NULL; wire the column through table traversal, expression deparsers and validation, and distinguish the newly declared constraint from an index. Includes construction/mutation, custom deparsing, metadata validation, legacy column nullability and round-trip tests plus usage documentation.

Validation: full Gradle check on Java 17; original and rendered DDL executed on PostgreSQL 18 for ALTER and CREATE forms, including quoted identifiers, NO INHERIT and combined ALTER actions. Verified the named ALTER constraint is recorded as contype=n, convalidated=false.

Syntax references: https://www.postgresql.org/docs/18/sql-createtable.html and https://www.postgresql.org/docs/18/sql-altertable.html