Add transaction() methods to MysqlClient

[jdickinsondev91-stack]

Add transaction() convenience method to MysqlClient

Currently, every application that needs atomic operations must manually issue START TRANSACTION, COMMIT, and ROLLBACK as raw queries and handle the rollback logic in a try/catch:

$mysql->query('START TRANSACTION');
try {
$mysql->query('INSERT INTO user (name) VALUES (?)', ['Alice']);
$mysql->query('INSERT INTO user (name) VALUES (?)', ['Bob']);
$mysql->query('COMMIT');
} catch (\Throwable $e) {
$mysql->query('ROLLBACK');
throw $e;
}

This PR adds a transaction() method that handles this automatically:

$mysql->transaction(function (React\Mysql\MysqlClient $mysql) {
$mysql->query('INSERT INTO user (name) VALUES (?)', ['Alice']);
$mysql->query('INSERT INTO user (name) VALUES (?)', ['Bob']);
});

The method:

• Starts a transaction before executing the callback
• Commits on success, rolls back on failure
• Handles both synchronous exceptions (fibers/await) and asynchronous promise rejections
• Returns a PromiseInterface consistent with the existing query() and ping() API

This pattern is standard in other database libraries (DB::transaction() in Laravel, transactional() in Doctrine, BEGIN/COMMIT wrappers in node-postgres and Go's database/sql).

Changes:

• src/MysqlClient.php — added transaction() method
• tests/MysqlClientTest.php — 5 unit tests
• README.md — usage documentation and table of contents
• CHANGELOG.md — unreleased entry