fix(parser): distinguish MySQL constraint and index names

[minleejae]

Summary

• distinguish the optional MySQL CONSTRAINT symbol from the optional index name
• support CONSTRAINT without a symbol and named UNIQUE or FOREIGN KEY indexes
• preserve getName() for the constraint symbol and expose the index name through getIndexName()
• retain USING and index comments during deparsing

These cases share the same constraint/index-name ambiguity and are implemented together.

Testing

• ./gradlew check

Fixes #1570
Fixes #538

AI assistance

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