fix: Auto-cancelled Sinks.Many would continue to accept emissions after cancellation

[georgebanasios]

Fixes #3715

Ensure auto-cancelled Sinks.Many reject emissions.

1.Fixed the WIP Leak in drain()
The core issue was that the drain() method had several premature return statements inside its main loop that were triggered on termination. These exits bypassed the logic that decrements the wip counter, effectively deadlocking the sink and preventing queue cleanup.
Change: All early return statements inside the for(; ; ) loop in drain() have been removed.
Effect: This guarantees the drain loop always completes its lifecycle, the wip counter is managed correctly, and the sink's serialization is always preserved.

2.Adjusted tryEmitNext() Against Races
Change: tryEmitNext() now performs a final isCancelled() check after offering an item to the queue and triggering a drain.
Effect: If a cancellation occurs concurrently with an emission, drain() cleans up the offered item, and tryEmitNext() correctly returns FAIL_CANCELLED. This prevents both item leaks and misleading OK results.

3.Corrected Auto-Cancellation Logic in remove()
Change: When the last subscriber is removed, the remove() method now simply sets the cancelled state and delegates all cleanup work to the drain() method.
Effect: This centralizes all state-change side effects (like clearing the queue) into the main serialized drain loop.