fix(parser): model MySQL column type modifiers

[minleejae]

Summary

• add structured SIGNED and UNSIGNED state to ColDataType
• model ZEROFILL consistently with and without numeric type arguments
• keep existing CAST(... AS UNSIGNED) forms working
• prevent column type modifiers from leaking into ColumnDefinition.columnSpecs

Testing

• ./gradlew check
• includes regression coverage for numeric column definitions and unsigned array casts

Fixes #2532

AI assistance

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