fix(parser): preserve unique index comments in ALTER TABLE

[minleejae]

Summary

• build the structured Index node for MySQL ALTER TABLE ... ADD UNIQUE INDEX
• retain key-part ordering, USING, and COMMENT clauses during parsing and deparsing
• keep the existing unique-key compatibility fields populated

Testing

• ./gradlew check

Fixes #2503

AI assistance

OpenAI Codex assisted with the implementation and test coverage. The changes were manually reviewed and verified.