windows: Improve foreground task dispatching on Windows (continued)

[JunkuiZhang]

Closes #22653 again

In PR #23283, I thought that every runnable dispatched to the main thread would correspond to an EVENT_DISPATCHED message in the message queue. However, after testing, some runnables occasionally weren’t executed.

This PR updated the code as follows:

if let Ok(runnable) = self.main_receiver.try_recv() {    <-- before
for runnable in self.main_receiver.drain() {            <-- after
    runnable.run();
}

This ensures that runnables are handled more proactively on the main thread, now we handle runnables with a much higher priority.

A big thanks to @MolotovCherry and @ArthurBrussee for their testing efforts!

Release Notes:

• N/A