fix: share MySQL character set parsing across column types and casts

[minleejae]

Valid MySQL casts and column definitions fail when they use the CHARSET abbreviation or quote the character set name. This also affects generated columns and ALTER TABLE MODIFY with ENUM types.

Route these forms through a shared ColDataType character-set production. Preserve the name's spelling and expose whether CHARSET was used; existing characterSet setters continue to render CHARACTER SET by default. CREATE TABLE, ALTER TABLE and CAST use the same path, with type equality/hashCode and AST mutation coverage.

This change addresses type character-set clauses. MySQL CONVERT(expression, type) is a separate existing grammar gap; the existing CONVERT(expression USING charset) form is covered as a control.

References: MySQL column character sets, CAST.

Validation:

• Full Gradle check passed on Java 17, including grammar ambiguity, formatting, static analysis, coverage and the full test suite. JUnit XML totals: 7822 tests, 0 failures/errors, 25 skipped.
• MySQL 8.4.11: 15 cases (12 valid, 3 invalid). For all 12 valid cases, the original SQL, toString output and StatementDeParser output executed with identical results.