A model's safety-filter refusal is an error that says so, not an empty reply

Opus's provider filter declined one of nisse-review's ask_expert
questions (which repositories a Gitea token can list) every time. It came
back as a 200 with no text, and the expert reported "unparseable reply:
empty reply". Both LLM clients now raise ModelRefused for
finish_reason content_filter (chat route) and stop_reason refusal
(passthrough). The expert's handback says what happened and what to do
instead.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-core/src/nisse_core/loop/expert.py

@@ -32,7 +32,7 @@ from nisse_core.log.canonical import (
     StoredEvent,
     UserMessage,
 )
-from nisse_core.models.llm import LLMClient, LLMError, RoleMap
+from nisse_core.models.llm import LLMClient, LLMError, ModelRefused, RoleMap
 from nisse_core.state.store import Transcript
 
 EXPERT_ROLE = "expert"
@@ -168,6 +168,9 @@ class ExpertDesk:
                     model=model,
                     system=system,
                 )
+            except ModelRefused as exc:
+                # Not retried: the same question is declined again.
+                return None, f"{exc}. Ask it differently, or work from the repository"
             except LLMError as exc:
                 return None, str(exc)
             text = "\n".join(