Replace O(n²) string concatenation of truncated_response_prefix in the
length-continuation retry loop with a list + ''.join(). Functionally
equivalent: same partial response on early return, same prepend on
final assembly. The legacy retry path is capped at 3 iterations, so
the practical wall-clock win is small, but the new idiom matches the
rest of the codebase and removes a needless repeated allocation.
Salvaged from PR #2717 (the run_conversation portion only — trajectory
refactor dropped because it silently rewrote </tool_response> to </think>).
Co-authored-by: Teknium <127238744+teknium1@users.noreply.github.com>
InB4DevOps
4f8aaf1046