fix(parser): model ALTER COLUMN nullability actions

[minleejae]

What

• Add a dedicated AlterExpression.ColumnSetNotNull AST model for ALTER COLUMN ... SET NOT NULL.
• Match the existing ColumnDropNotNull production before the generic column-data-type production.
• Preserve both actions through AlterExpression.toString().
• Validate the affected column name in AlterValidator.
• Add AST, parse/deparse, validation, and reflection coverage.

Why

Both statements currently parse, but their ASTs are not reliable:

ALTER TABLE test_table ALTER COLUMN name SET NOT NULL;
ALTER TABLE test_table ALTER COLUMN name DROP NOT NULL;

SET NOT NULL is represented as a column data type named SET with [NOT, NULL] in its raw column specs. DROP NOT NULL is also consumed by the generic column-data-type branch even though a dedicated ColumnDropNotNull model already exists. Consumers therefore have to inspect and reconstruct raw tokens instead of using the AST.

This change gives both PostgreSQL nullability actions explicit AST representations without changing unrelated column option parsing.

PostgreSQL grammar reference: https://www.postgresql.org/docs/current/sql-altertable.html

Testing

• ./gradlew check
• Added parse/deparse and AST assertions for both actions.
• Added validator coverage for SET NOT NULL.
• Added the new model to ReflectionModelTest.

AI assistance

OpenAI Codex was used to trace the grammar branch selection, prepare the AST/deparser/validator changes, and run validation. The generated change and tests were reviewed against the PostgreSQL documentation.

[manticore-projects]

Thanks!