Model MySQL ALTER TABLE partition operations

[minleejae]

Completes the remaining ALTER TABLE half of #1668 after #2521.

• returns AlterExpressionPartition for every MySQL partition operation
• reuses TablePartitioning for structured PARTITION BY access
• models ALL, coalesce count, exchange table, and validation mode explicitly
• fixes WITHOUT VALIDATION and supports quoted partition/table names
• covers the MySQL 8.4 partition-operation grammar with round-trip and AST tests

./gradlew check passes.

Closes #1668