fix: append_session inner loop while condition

[sgbalogh]

Resolves #90

Reproduces with this:

rand_key=$(head -c 8 /dev/urandom | base64) && echo $rand_key
count=1; while [ "$count" -le 1000 ] ; do echo "$count $rand_key"; ((count++)); done \
        | s2 stream my-basin4 my-stream1 append

which returns:

sbFi2e7W3iw=
Error:
  × Main thread panicked.
  ├─▶ at /Users/sb/.cargo/git/checkouts/s2-sdk-rust-6c37de187e75d8b0/d40de3d/src/append_session.rs:293:5
  ╰─▶ assertion `left == right` failed
        left: 1
       right: 0
  help: set the `RUST_BACKTRACE=1` environment variable to display a backtrace.

When version of SDK used by CLI is updated, appends work as expected.

[shikhar]

description vs code change ratio :D