[minleejae]

ALTER TABLE ADD parses table constraints with the CREATE TABLE production only for Dialect.MYSQL and Dialect.POSTGRESQL (#2612, #2654). Without either dialect, the generic ALTER branches reject forms that CREATE TABLE accepts in every dialect:

• ALTER TABLE t ADD CHECK (c > 0)
• ALTER TABLE t ADD CONSTRAINT ck CHECK (c > 0) NOT VALID
• ALTER TABLE t ADD FOREIGN KEY (id) REFERENCES parent (id) DEFERRABLE INITIALLY DEFERRED
• ALTER TABLE t ADD CONSTRAINT pk PRIMARY KEY (id) DEFERRABLE INITIALLY DEFERRED
• ALTER TABLE t ADD CONSTRAINT ex EXCLUDE USING gist (r WITH &&)

An ADD now uses the shared production in every dialect when it contains a form that only that production accepts: an unnamed CHECK, FOREIGN KEY, EXCLUDE, or INITIALLY IMMEDIATE|DEFERRED, NOT VALID or ENFORCED outside a CHECK. Statements that already parsed keep their generic representation, accessors and output. The PostgreSQL-only UNIQUE USING INDEX and NOT NULL constraints still require Dialect.POSTGRESQL. The scan lives in the package-private AlterConstraintLookahead, like RoutineBodyBoundary, so that it adds little to CCJSqlParser.

The second commit makes VALIDATE CONSTRAINT a structured RelationAlterAction in every dialect. Outside PostgreSQL it was an UNSPECIFIC expression, so its constraint name was not accessible. Its output is unchanged.

Existing parser result classes and public method signatures are retained.

Syntax reference: PostgreSQL 18 ALTER TABLE.

Validation:

• AlterAddTableConstraintTest compares CREATE TABLE and ALTER ADD for eight definitions without a dialect and with ANSI_SQL, ORACLE, SQLSERVER and H2, and pins the representation of forms that already parsed. 10 of its 13 cases fail on master. PostgreSqlRelationAlterTest covers VALIDATE CONSTRAINT without the PostgreSQL dialect.
• Removing the dialect test from the gate instead failed 24 existing tests (Oracle constraint states, USING INDEX, legacy key accessors), so only forms that do not parse today are rerouted.
• Java 17 Gradle check: 12,091 tests, 0 failures, 25 skipped; grammar ambiguity, formatting (Spotless ratchet against upstream/master), Checkstyle, PMD, SpotBugs and JaCoCo pass.
• PostgreSQL 18.3: the statements above, VALIDATE CONSTRAINT and the PR's other cases (17 statements) give the same results for the original SQL and for the SQL rendered without a dialect and with Dialect.POSTGRESQL.