contentLengthStream

    fn contentLengthStream(
        io_r: *std.Io.Reader,
        w: *Writer,
        limit: std.Io.Limit,
    ) std.Io.Reader.StreamError!usize {
        const reader: *Reader = @alignCast(@fieldParentPtr("interface", io_r));
        if (reader.state == .ready) return error.EndOfStream;
        const remaining_content_length = &reader.state.body_remaining_content_length;
        const remaining = remaining_content_length.*;
        const n = try reader.in.stream(w, limit.min(.limited64(remaining)));
        if (n == remaining) {
            reader.state = .ready;
        } else {
            remaining_content_length.* = remaining - n;
        }
        return n;
    }