fix: reuse table constraints for MySQL ALTER TABLE

[minleejae]

MySQL ALTER TABLE ADD rejects constraint definitions that already work in CREATE TABLE, including CONSTRAINT uq UNIQUE KEY idx (id) and unnamed CHECK constraints. With Dialect.MYSQL, both paths now use the existing table-constraint parser and preserve the constraint symbol and index name separately.

Reuse NamedConstraint/CheckConstraint instead of extending the legacy ALTER branches. Keep named-index options on their AST, preserve legacy key accessors, and require one CHECK expression. Tests cover quoted and omitted names, KEY/INDEX variants, enforcement, multiple ALTER actions, AST edits, and custom expression deparsing; the existing PostgreSQL, Informix, and SQL Server tests also pass.

Syntax reference: MySQL 8.4 ALTER TABLE.

Validation:

• Java 17 Gradle check: 7,826 XML-reported tests, 0 failures/errors, 25 skipped; formatting, grammar ambiguity, Checkstyle, PMD, and JMH compilation passed. Formatting was checked against upstream/master using a local Spotless ratchet override.
• Actual MySQL 8.4.11 server: 12 accepted and 4 rejected SQL cases matched the expected behavior.
• Executed both AST and StatementDeParser output for the accepted cases on the same server: 24 successful executions.

[manticore-projects]

I am really grateful for this consolidation and cleanup, thank you!