fix: event parsing iterator

The EventParsingIterator implementation does not parse correctly events when their payload do not have the eventpayload property set. The current behavior is that if said property "eventpayload" is not present then it fallback to extracting the value from the headers, but this is not a reliable method for parsing event streams since seems that not all services uses this pattern to provide the events. One example is the InvokeModelWithResponseStream operation from bedrock-runtime service. To fix this we pretty much followed the specs found in the smithy reference: https://smithy.io/2.0/spec/streaming.html#eventpayload-trait#id6, which explains that when eventpayload and eventheader is not set for any of the members of the shape then, the payload  itself should be considered as the top level structured to be parsed.

src/Api/Parser/EventParsingIterator.php

@@ -69,36 +69,43 @@ private function parseEvent(array $event)
             if ($event['headers'][':message-type'] === 'error') {
                 return $this->parseError($event);
             }
+
             if ($event['headers'][':message-type'] !== 'event') {
                 throw new ParserException('Failed to parse unknown message type.');
             }
         }
 
-        if (empty($event['headers'][':event-type'])) {
+        $eventType = $event['headers'][':event-type'] ?? null;
+        if (is_null($eventType)) {
             throw new ParserException('Failed to parse without event type.');
         }
-        $eventShape = $this->shape->getMember($event['headers'][':event-type']);
 
+        $eventShape = $this->shape->getMember($eventType);
+        $eventPayload = $event['payload'];
         $parsedEvent = [];
-        foreach ($eventShape['members'] as $shape => $details) {
-            if (!empty($details['eventpayload'])) {
-                $payloadShape = $eventShape->getMember($shape);
-                if ($payloadShape['type'] === 'blob') {
-                    $parsedEvent[$shape] = $event['payload'];
+        foreach ($eventShape['members'] as $eventShapeMemberKey => $eventShapeMemberProps) {
+            $memberShape = $eventShape->getMember($eventShapeMemberKey);
+            if (!empty($eventShapeMemberProps['eventheader'])) {
+                $parsedEvent[$eventShapeMemberKey] = $event['headers'][$eventShapeMemberKey];
+            } elseif (!empty($eventShapeMemberProps['eventpayload'])) {
+                if ($memberShape['type'] === 'blob') {
+                    $parsedEvent[$eventShapeMemberKey] = $eventPayload;
                 } else {
-                    $parsedEvent[$shape] = $this->parser->parseMemberFromStream(
-                        $event['payload'],
-                        $payloadShape,
+                    $parsedEvent[$eventShapeMemberKey] = $this->parser->parseMemberFromStream(
+                        $eventPayload,
+                        $memberShape,
                         null
                     );
                 }
-            } else {
-                $parsedEvent[$shape] = $event['headers'][$shape];
             }
         }
 
+        if (empty($parsedEvent) && !empty($eventPayload->getContents())) {
+           $parsedEvent = $this->parser->parseMemberFromStream($eventPayload, $eventShape, null);
+        }
+
         return [
-            $event['headers'][':event-type'] => $parsedEvent
+            $eventType => $parsedEvent
         ];
     }
 
@@ -109,4 +116,4 @@ private function parseError(array $event)
             $event['headers'][':error-message']
         );
     }
-}
+}