[FIX] Prevent zero-size dump batches for small tables.

[middleDuckAi]

Closes #2288

What changed

• prevent MysqlDumper from producing a zero row batch size when a table has only a few rows but is split into multiple parts
• move the batch-size math into a small helper for safer regression coverage
• add a regression test for the reported divide-by-zero shape

Why

round($rowCount / $parts) can become 0 for small positive row counts, which then causes a division-by-zero crash when calculating the total page count.

Validation

• php -l core/src/Support/MysqlDumper.php
• php -l core/tests/Unit/Support/MysqlDumperRowBatchSizeTest.php
• cd core && ./vendor/bin/pest tests/Unit/Support/MysqlDumperRowBatchSizeTest.php tests/Feature/ModifiersTest.php