Returning ServiceBusMessage doesn't work correctly

When returning a `ServiceBusMessage`, the message posted contains incorrect information.

```
[Function(nameof(Run))]
[ServiceBusOutput("audit", Connection = "AzureServiceBus")]
public async Task Run([ServiceBusTrigger("Notifications", Connection = "AzureServiceBus")] ServiceBusReceivedMessage message)
{
// removed for brevity

return new ServiceBusMessage(rawMessage);
}
```

For an incoming message


The returned message body is

1 - missing original header
2 - incorrect body
3 - incorrect content type

## Expected:


1 - the original header (`EventType`) is found
2 - original body
3 - correct content type

The report provides a C# reproducer through Run using ServiceBusMessage, ServiceBusOutput, ServiceBusTrigger, and ServiceBusReceivedMessage. Start by running that flow and tracing how the output binding converts the returned message. Done means the posted message preserves the original EventType header, body, and content type shown in the expected result.

Written by the indexing model from the issue text.