Add global main coroutine switch handlers API for context isolation

  Introduces API for registering global switch handlers that are automatically
  attached to the main coroutine when the scheduler starts lazily. This enables
  modules to set up proper context isolation (like output buffers) before any
  coroutine execution begins.

  Key features:
  - Global handler registration with persistent memory allocation
  - Automatic handler attachment to main coroutine on scheduler startup
  - Proper cleanup during PHP shutdown
  - Thread-safe execution protection

  API functions:
  - zend_async_add_main_coroutine_start_handler()
  - zend_async_call_main_coroutine_start_handlers()
  - Corresponding ZEND_ASYNC_ADD_MAIN_COROUTINE_START_HANDLER macros

  This solves the chicken-and-egg problem where modules need to initialize
  context before the scheduler creates the main coroutine, enabling proper
  isolation of global state like output buffers between coroutines.

Author: EdmondDantes

coroutine.c

@@ -155,6 +155,14 @@ void async_coroutine_finalize(zend_fiber_transfer *transfer, async_coroutine_t *
 {
 	ZEND_COROUTINE_SET_FINISHED(&coroutine->coroutine);
 
+	/* Call switch handlers for coroutine finishing */
+	if (UNEXPECTED(coroutine->coroutine.switch_handlers)) {
+		ZEND_COROUTINE_FINISH(&coroutine->coroutine);
+	}
+
+	/* Cleanup switch handlers */
+	zend_coroutine_switch_handlers_destroy(&coroutine->coroutine);
+
 	// call coroutines handlers
 	zend_object * exception = NULL;
 
@@ -241,6 +249,11 @@ ZEND_STACK_ALIGNED void async_coroutine_execute(zend_fiber_transfer *transfer)
 	async_coroutine_t *coroutine = (async_coroutine_t *) ZEND_ASYNC_CURRENT_COROUTINE;
 	ZEND_COROUTINE_SET_STARTED(&coroutine->coroutine);
 
+	/* Call switch handlers for coroutine entering */
+	if (UNEXPECTED(&coroutine->coroutine.switch_handlers != NULL)) {
+		ZEND_COROUTINE_CALL_SWITCH_HANDLERS(&coroutine->coroutine, true, false);
+	}
+
 	/* Determine the current error_reporting ini setting. */
 	zend_long error_reporting = INI_INT("error_reporting");
 	if (!error_reporting && !INI_STR("error_reporting")) {
@@ -609,6 +622,9 @@ static zend_object *coroutine_object_create(zend_class_entry *class_entry)
 	ZEND_ASYNC_EVENT_SET_NO_FREE_MEMORY(&coroutine->coroutine.event);
 	ZEND_ASYNC_EVENT_SET_ZEND_OBJ_OFFSET(&coroutine->coroutine.event, XtOffsetOf(async_coroutine_t, std));
 
+	/* Initialize switch handlers */
+	coroutine->coroutine.switch_handlers = NULL;
+
 	zend_async_event_t *event = &coroutine->coroutine.event;
 
 	event->start = coroutine_event_start;

scheduler.c

@@ -739,27 +739,34 @@ void async_scheduler_coroutine_suspend(zend_fiber_transfer *transfer)
 
 	ZEND_ASYNC_SCHEDULER_HEARTBEAT;
 
+	zend_coroutine_t * coroutine = ZEND_ASYNC_CURRENT_COROUTINE;
+
 	//
 	// Before suspending the coroutine,
 	// we start all its Waker-events.
 	// This causes timers to start, POLL objects to begin waiting for events, and so on.
 	//
-	if (transfer == NULL && ZEND_ASYNC_CURRENT_COROUTINE != NULL && ZEND_ASYNC_CURRENT_COROUTINE->waker != NULL) {
-		async_scheduler_start_waker_events(ZEND_ASYNC_CURRENT_COROUTINE->waker);
+	if (transfer == NULL && coroutine != NULL && coroutine->waker != NULL) {
+		async_scheduler_start_waker_events(coroutine->waker);
 
 		// If an exception occurs during the startup of the Waker object,
 		// that exception belongs to the current coroutine,
 		// which means we have the right to immediately return to the point from which we were called.
 		if (UNEXPECTED(EG(exception) != NULL)) {
 			// Before returning, We are required to properly destroy the Waker object.
 			zend_exception_save();
-			async_scheduler_stop_waker_events(ZEND_ASYNC_CURRENT_COROUTINE->waker);
-			zend_async_waker_destroy(ZEND_ASYNC_CURRENT_COROUTINE);
+			async_scheduler_stop_waker_events(coroutine->waker);
+			zend_async_waker_destroy(coroutine);
 			zend_exception_restore();
 			return;
 		}
 	}
 
+	if (UNEXPECTED(coroutine->switch_handlers)) {
+		ZEND_COROUTINE_LEAVE(coroutine);
+		ZEND_ASSERT(EG(exception) == NULL && "The exception after ZEND_COROUTINE_LEAVE must be NULL");
+	}
+
 	//
 	// The async_scheduler_coroutine_suspend function is called
 	// with the transfer parameter not null when the current coroutine finishes execution.
@@ -830,6 +837,10 @@ void async_scheduler_coroutine_suspend(zend_fiber_transfer *transfer)
 	} else {
 		switch_to_scheduler(transfer);
 	}
+
+	if (UNEXPECTED(coroutine->switch_handlers && transfer == NULL)) {
+		ZEND_COROUTINE_ENTER(coroutine);
+	}
 }
 
 void async_scheduler_main_loop(void)