Incomplete receive data is delivered as a complete WebSocket message

Start at NWWebSocket.listen(connection:generation:) and inspect the NWConnection.receiveMessage callback, including its data, context, isComplete, and error values. Add regression coverage for complete, incomplete, empty, and missing-context receives, then verify that only complete data reaches the delegate while transport errors still stop the receive loop.

Problem

NWWebSocket.listen(connection:generation:) ignores the isComplete value from NWConnection.receiveMessage.

The completion can contain non-empty data, isComplete == false, and a transport error. The current code sends that data to receiveMessage(data:context:) before it handles the error.

A text delegate can therefore receive an incomplete UTF-8 or JSON message as a complete WebSocket message.

Apple documents receiveMessage as a complete-message receive API and provides isComplete in the callback:
https://developer.apple.com/documentation/network/nwconnection/receivemessage(completion:)

Current behavior

connection?.receiveMessage { [weak self] (data, context, _, error) in
    // ...
    if let data = data, !data.isEmpty, let context = context {
        self.receiveMessage(data: data, context: context)
    }
    // The transport error is handled after data delivery.
}

Expected behavior

The library sends data to its delegate only when isComplete is true.

The existing error path continues to report the transport failure and stop the receive loop.

Proposed fix

• Read the isComplete callback value.
• Require isComplete before message delivery.
• Add regression tests for complete, incomplete, empty, and missing-context receive values.

I searched the existing issues and pull requests for isComplete, receiveMessage, partial data, incomplete data, truncated data, and fragments. I found no report for this case.