Function detect_stream_type 

fn detect_stream_type(first_byte: u8) -> DataStreamType

Determine the data stream type from the first varint on the stream.

MoQT data streams start with a stream type varint:

• 0x04 = Subgroup
• 0x05 = Fetch

Returns (stream_type, bytes_consumed) so the caller can account for the type varint when setting up the parser.