Function yield_once_callback

tests/test-scheduler-basic-thorough.cpp:328–330

* @brief Callback for yield-once testing * * Records execution for yield-once verification. */

 * Records execution for yield-once verification.
 */
void yield_once_callback() {
    test_output.push_back("yield_once_executed");
}

// Test fixture for comprehensive scheduler testing
class SchedulerThoroughTest : public ::testing::Test {