[ObjC] fix error handling in CFWriteStreamWrite (#37344)

follow up of #37255

Closes #37344

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37344 from HannahShiSFB:cfstream-write-error-handling c9a751d9d7
PiperOrigin-RevId: 663329438

src/core/lib/event_engine/cf_engine/cfstream_endpoint.cc

@@ -335,9 +335,18 @@ void CFStreamEndpointImpl::DoWrite(
       continue;
     }
 
-    size_t written_size =
+    CFIndex written_size =
         CFWriteStreamWrite(cf_write_stream_, slice.begin(), slice.size());
 
+    if (written_size < 0) {
+      auto status = CFErrorToStatus(CFWriteStreamCopyError(cf_write_stream_));
+      GRPC_TRACE_LOG(event_engine_endpoint, INFO)
+          << "CFStream write error: " << status
+          << ", written_size: " << written_size;
+      on_writable(status);
+      return;
+    }
+
     total_written_size += written_size;
     if (written_size < slice.size()) {
       SliceBuffer written;