test(transports): lock in content_filter passthrough for OpenRouter

OpenRouter (and every other OpenAI-compatible provider) uses the default
chat_completions transport, so it is already covered by the refusal fix:
an upstream Claude / moderation refusal arrives as
finish_reason="content_filter" (often empty content, no message.refusal).
Add a regression test asserting the transport passes that finish reason
straight through to the loop's content_filter handler.

(cherry picked from commit 60168a513b)

tests/agent/transports/test_chat_completions.py

@@ -900,6 +900,28 @@ class TestChatCompletionsNormalize:
         assert nr.content == "declined"
         assert nr.provider_data == {"refusal": "declined"}
 
+    def test_explicit_content_filter_finish_reason_passes_through(self, transport):
+        """OpenRouter (and other OpenAI-compatible providers) surface an
+        upstream Claude / moderation refusal as ``finish_reason="content_filter"``
+        — often with empty content and no ``message.refusal`` field. The
+        transport must pass that finish reason straight through so the loop's
+        content_filter refusal handler fires; no ``message.refusal`` required.
+        This is the OpenRouter coverage path (OpenRouter uses the default
+        chat_completions transport)."""
+        r = SimpleNamespace(
+            choices=[SimpleNamespace(
+                message=SimpleNamespace(
+                    content=None, tool_calls=None, reasoning_content=None,
+                    refusal=None,
+                ),
+                finish_reason="content_filter",
+            )],
+            usage=None,
+        )
+        nr = transport.normalize_response(r)
+        assert nr.finish_reason == "content_filter"
+        assert nr.content is None
+
     def test_refusal_does_not_clobber_existing_content(self, transport):
         """If the model emitted partial text *and* a refusal, keep the visible
         text as content but still flag the refusal via content_filter."""