fix(grpc): drain log stream gracefully on success path in solve_lp/mip

stop_log_streaming() calls TryCancel() which races against the server's
final log drain: poll_for_completion() returns as soon as the job is
marked complete, then TryCancel() kills the stream before the server has
finished sending remaining lines (e.g. "Best objective …").

Add drain_log_streaming(): polls log_stream_done_ (set when the thread
exits naturally after receiving the job_complete sentinel) for up to 5s,
then falls back to stop_log_streaming(). Use it on the success path in
solve_lp and solve_mip so all final log lines are received before the
stream is torn down.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Ramakrishna Prabhu <ramakrishnap@nvidia.com>

Author: ramakrishnap-nv

cpp/src/grpc/client/grpc_client.cpp

@@ -319,6 +319,7 @@ void grpc_client_t::drain_log_streaming()
   // common non-streaming solve path.
   if (!log_thread_) return;
 
+
   // On the success path the server drains all pending log lines and then sends
   // a job_complete sentinel; the streaming thread exits naturally when that
   // sentinel arrives.  Poll for natural completion (up to kDrainTimeout) so