PostgreSQL: COMMENT ON CONSTRAINT / OPERATOR / RULE

[fmguerreiro]

Summary

Adds parser + AST coverage for the three remaining COMMENT ON … shapes that the fork had been silently rejecting:

• COMMENT ON CONSTRAINT name ON [DOMAIN] target IS '…'
• COMMENT ON OPERATOR name (left, right) IS '…' — each side may be NONE for unary operators
• COMMENT ON RULE name ON target IS '…'

Changes

• CommentObject gains Constraint, Operator, Rule variants in keyword_str and from_keyword.
• New CommentOperatorArgs { left: Option<DataType>, right: Option<DataType> } carries the operand signature for OPERATOR, with Display rendering NONE for unary slots.
• Statement::Comment gains two fields: operator_args: Option<CommentOperatorArgs> and on_domain: bool (only meaningful for Constraint). Existing destructures will need to add the two fields.
• parse_comment dispatches the new variants:
  • OPERATOR uses parse_operator_name() (so +, myschema.@@, etc. round-trip) and a mandatory (slot, slot) signature.
  • CONSTRAINT and RULE require an ON <relation> tail; CONSTRAINT additionally accepts ON DOMAIN <domain>.
• Refactors parse_drop_operator_signature to share a new parse_operator_arg_type_or_none() helper with parse_comment.
• Bumps fork to 0.63.0; adds changelog/0.63.0.md.

Breaking changes

Statement::Comment adds operator_args and on_domain fields. Consumers that destructure all fields explicitly must add them; those using .. rest patterns are unaffected. Documented in changelog/0.63.0.md.

Test plan

• cargo test --package pgmold-sqlparser (CI)
• New tests in tests/sqlparser_postgres.rs:
  • parse_comment_on_constraint_on_table (binary form + IS NULL + IF EXISTS)
  • parse_comment_on_constraint_on_domain (unqualified + schema-qualified domain)
  • parse_comment_on_constraint_requires_relation_tail (error case)
  • parse_comment_on_operator_binary (public.+(INTEGER, INTEGER))
  • parse_comment_on_operator_prefix_left_none (unary prefix, NONE left)
  • parse_comment_on_operator_postfix_right_none (unary postfix, NONE right)
  • parse_comment_on_operator_requires_argument_list (error case)
  • parse_comment_on_rule + IS NULL
  • parse_comment_on_rule_requires_relation_tail (error case)
• Updated all existing Statement::Comment destructures in tests/sqlparser_postgres.rs to include the two new fields.