feat: add Query Builder lock wait modifiers

[memleakd]

Description

This PR adds nowait() and skipLocked() methods to Query Builder for pessimistic SELECT locks.

They can be used together with lockForUpdate() or sharedLock() to control what should happen when selected rows cannot be locked immediately.

Example:

$db->transaction(static function ($db): void {
    $jobs = $db->table('jobs')
        ->where('status', 'pending')
        ->orderBy('id', 'ASC')
        ->limit(10)
        ->lockForUpdate()
        ->skipLocked()
        ->get()
        ->getResultArray();

    // Mark or update the claimed jobs before committing the transaction...
});

Unsupported combinations throw a DatabaseException, the same way existing pessimistic lock limitations are handled.

Supported combinations:

• lockForUpdate()->nowait(): MySQLi, Postgre, OCI8, SQLSRV
• lockForUpdate()->skipLocked(): MySQLi, Postgre, OCI8, SQLSRV
• sharedLock()->nowait(): Postgre, SQLSRV
• sharedLock()->skipLocked(): Postgre

SQLSRV uses table hints for this feature. For skipLocked(), it uses READCOMMITTEDLOCK and READPAST, with the documented SQL Server limitation that some locks may still be waited on.

Tests were added for generated SQL, unsupported combinations, reset behavior, and exists() / countAllResults() handling.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide