fix(async_context_threadsafe_background_execute_sync): prevent use-after-return by removing worker before semaphore release

Ensures the async worker is unregistered from the context before releasing the
blocking thread. This prevents the `sync_func_call_t` stack object from being
accessed after it goes out of scope, avoiding a use-after-return condition
in multicore `async_context_execute_sync()` scenarios.

Related to raspberrypi/pico-sdk#2433

Author: schkovich