fix(agent): re-enter retry loop on genuine Nous 429 so fallback guard runs

The genuine-rate-limit branch set retry_count = max_retries before
continue, intending the top-of-loop Nous guard to handle fallback or
bail cleanly. But the loop condition is retry_count < max_retries, so
the guard never ran: no fallback activation, no clean rate-limit
message — just the generic retry-exhaustion error.

Set retry_count = max(0, max_retries - 1) so the loop body runs exactly
once more and the guard sees the breaker state recorded moments earlier.

Extracted from the #44061 bugfix rollup by @AIalliAI.

agent/conversation_loop.py

@@ -2631,10 +2631,13 @@ def run_conversation(
                     except Exception:
                         pass
                     if _genuine_nous_rate_limit:
-                        # Skip straight to max_retries -- the
-                        # top-of-loop guard will handle fallback or
-                        # bail cleanly.
-                        retry_count = max_retries
+                        # Re-enter the loop exactly once so the
+                        # top-of-loop Nous guard handles fallback or
+                        # bails cleanly. (Setting retry_count to
+                        # max_retries would make the while condition
+                        # false immediately and the guard would never
+                        # run -- no fallback, generic exhaustion error.)
+                        retry_count = max(0, max_retries - 1)
                         continue
                     # Upstream capacity 429: fall through to normal
                     # retry logic.  A different model (or the same