Fix the frame extractor for a TCP stream

Your internal RPC protocol sends length-prefixed messages over TCP. The first reader assumed one read returns one whole message. TCP does not work that way: a read can return half a message, or several at once. The fix is a framing layer that pulls every complete frame out of whatever bytes have arrived.

An assistant wrote extract_frames(stream, max_frame_len). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• stream: the bytes received so far, as a list of integers from 0 to 255.
• max_frame_len: the largest payload a frame may declare.
• Return the payloads of the complete frames, in order, each as a list of integers.

Rules the tests check:

• A frame is a 4-byte length, then that many payload bytes. The length is big-endian: the first byte is the most significant.
• Read frames from the start of stream. Stop as soon as one of these holds:
  • fewer than 4 bytes remain for the next length,
  • the declared length is greater than max_frame_len,
  • fewer bytes remain than the declared length.
• Return the payloads of the frames read before stopping. A length of 0 is a complete frame with an empty payload.
• Bytes after the stopping point wait for the next read, so they are never returned.