Fix PHP 8.4 task exit handling

[realFlowControl]

Fixes a PHP 8.4 regression where a task that calls exit after worker-side includes could prevent a parallel\Runtime from continuing to drain queued tasks.

On PHP 8.4 and newer, exit/die reaches the task scheduler as PHP’s internal UnwindExit/GracefulExit exception. The scheduler was treating that object as a normal task exception and passing it to php_parallel_exceptions_save(), which expects normal Error/Throwable properties. This shuts down the runtime and as such will not execute scheduled tasks.

This change detects the internal exit objects before the normal exception-copy path, clears them, marks only the current future as killed, and lets the worker continue processing queued work.

Fixes #375