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/tests/test_expert.py

@@ -18,7 +18,7 @@ from nisse_core.loop.expert import (
     ExpertDesk,
     parse_answer,
 )
-from nisse_core.models.llm import LLMError, RoleMap
+from nisse_core.models.llm import LLMError, ModelRefused, RoleMap
 from nisse_core.oplog import OpLog
 from nisse_core.state.store import Transcript
 from nisse_dev.state import StateStore
@@ -142,6 +142,29 @@ def test_unparseable_reply_is_retried_once_then_raises(store: StateStore) -> Non
     assert "not a single valid JSON object" in client.calls[1]["system"]
 
 
+class DecliningClient(ScriptedClient):
+    """The provider's safety filter, declining every time."""
+
+    def complete(self, events: Any, tools: Any, model: str, system: Any = None) -> Any:
+        self.calls.append({"model": model, "system": system})
+        raise ModelRefused(f"{model} declined to answer: its provider's safety filter")
+
+
+def test_a_declined_question_is_not_asked_again_and_says_why(
+    store: StateStore,
+) -> None:
+    client = DecliningClient([])
+    with pytest.raises(LLMError) as raised:
+        desk(client, store).ask("which repositories can a token list?")
+    assert len(client.calls) == 1  # the same question is declined again
+    assert "Ask it differently, or work from the repository" in str(raised.value)
+    [completed] = [
+        e.event for e in log(store).read() if isinstance(e.event, DelegationCompleted)
+    ]
+    assert completed.exit_status == "failed"
+    assert "safety filter" in completed.handback
+
+
 def test_retry_succeeds_after_a_bad_first_reply(store: StateStore) -> None:
     client = ScriptedClient(["waffle", GOOD])
     assert desk(client, store).ask("q").confidence == "high"