feat: Add transactional method to manage closures within transactions

[marcreichel]

Summary

Adds a transactional() method to Connection that runs a Closure inside a database transaction, committing on success and rolling back on any Throwable before rethrowing. This removes the need for callers to manually pair beginTransaction() / commit() / rollBack() around their logic.

Changes

• src/Connection.php: Implements transactional(Closure $callback): mixed. Begins a transaction, invokes the callback, rolls back and rethrows on Throwable, otherwise commits and returns the callback's result.
• src/DoctrineConnectionInterface.php: Adds the transactional() method to the interface with a generic @template TReturn annotation so the return type is preserved for static analysis.
• src/MockConnection.php: Provides a no-op implementation that simply invokes the callback (no real transaction handling needed in tests).

Usage

$result = $connection->transactional(function () use ($connection) {
    $connection->insert(...);
    $connection->update(...);

    return $someValue;
});