Add Fiber::getCoroutine() method

Provides access to the underlying coroutine object when a fiber
is running inside an async coroutine. Returns null for regular fibers.

This allows using all coroutine methods (getId, getContext, cancel, etc.)
without duplicating the API in Fiber class.

Example usage:
    $fiber = new Fiber(fn() => /* ... */);
    $fiber->start();
    $coroutine = $fiber->getCoroutine();
    $id = $coroutine->getId();
    $context = $coroutine->getContext();

Author: Edmond

Zend/zend_fibers.c

@@ -1794,6 +1794,20 @@ ZEND_METHOD(Fiber, getCurrent)
 	RETURN_OBJ_COPY(&fiber->std);
 }
 
+ZEND_METHOD(Fiber, getCoroutine)
+{
+	ZEND_PARSE_PARAMETERS_NONE();
+
+	zend_fiber *fiber = (zend_fiber *) Z_OBJ_P(ZEND_THIS);
+
+	if (fiber->coroutine == NULL) {
+		RETURN_NULL();
+	}
+
+	zend_object *coroutine_obj = ZEND_ASYNC_EVENT_TO_OBJECT(&fiber->coroutine->event);
+	RETURN_OBJ_COPY(coroutine_obj);
+}
+
 ZEND_METHOD(FiberError, __construct)
 {
 	zend_throw_error(

Zend/zend_fibers.stub.php

@@ -26,6 +26,8 @@ public function isTerminated(): bool {}
 
     public function getReturn(): mixed {}
 
+    public function getCoroutine(): ?\Async\Coroutine {}
+
     public static function getCurrent(): ?Fiber {}
 
     public static function suspend(mixed $value = null): mixed {}