Function parse_message 

pub fn parse_message(
    buf: &[u8],
    start: usize,
    my_group: Option<u64>,
    schema: Option<&MessageDescriptor>,
    full_schema: &ParsedSchema,
    annotations: bool,
) -> (ProtoTextMessage, usize, Option<WiretagResult>)

Parse one protobuf message starting at start.

Returns (message, next_pos, group_end_tag). group_end_tag is Some(tag) when the parse terminated on an END_GROUP wire type — the caller uses this to detect mismatched group numbers.

Mirrors parse_message() in decode.py.