fix(loop): prevent step_idx advance on unhandled exceptions + fix consecutive assistant messages

Two bugs that together caused an infinite SKIP loop after LoopResumeError:

1. loop.py _run_step: set step_forward=False in the `else: raise` branch so that
   when LoopResumeError propagates from _propose (LLMUnavailableError), step_idx
   stays at 0. Previously it advanced to 1, leaving loops permanently stuck with
   missing direct_exp_gen result on next resume.

2. base.py _create_chat_completion_auto_continue: when finish_reason=="length"
   triggers a continuation retry, merge into the previous assistant message instead
   of appending a second consecutive one. llama-server returns 400 on two consecutive
   assistant messages, which caused LLMUnavailableError -> LoopResumeError cascade.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rdagent/utils/workflow/loop.py

@@ -270,6 +270,11 @@ class LoopBase:
                         msg = "We have reset the loop instance, stop all the routines and resume."
                         raise self.LoopResumeError(msg) from e
                     else:
+                        # Do NOT advance step_idx for unhandled exceptions (e.g. LoopResumeError
+                        # propagating from _propose). Keeping step_idx at the current step lets
+                        # kickoff_loop retry step 0 on the next resume instead of permanently
+                        # corrupting the loop with a missing direct_exp_gen result.
+                        step_forward = False
                         raise  # re-raise unhandled exceptions
                 finally:
                     # No matter the execution succeed or not, we have to finish the following steps