Reuse PostgreSQL constraint attributes in ALTER CONSTRAINT

[minleejae]

PostgreSQL ALTER CONSTRAINT accepts deferrability, initial timing, and enforcement attributes, including combinations that the existing ALTER parser rejected. CREATE TABLE and ADD CONSTRAINT already had a structured representation for these attributes.

Reuse ConstraintAttributes in a dedicated ALTER CONSTRAINT node, retaining the existing constraint-name and enforcement accessors. Share attribute parsing with CREATE/ADD constraints and reject empty ALTER actions, duplicate attributes, and NOT VALID in this context.

Tests compare CREATE, ADD and ALTER representations, exercise attribute permutations and AST mutations, and retain the existing ENFORCED behavior. Relevant accepted cases were executed on PostgreSQL 18.6, with original and both rendered forms producing matching constraint catalog state. Constraint-type/catalog restrictions remain the database's responsibility.

Syntax reference: ALTER TABLE.

Validation: focused tests, JavaCC ambiguity checks, Checkstyle and Spotless on this independent branch. The combined implementation also passes the full Gradle check and Maven verify; the 1,304-input prior regression corpus has no previously successful input regressions.