fix(fallback): merge fallback_providers with legacy fallback_model configurations

tests/gateway/test_auth_fallback.py

@@ -71,3 +71,46 @@ class TestResolveRuntimeAgentKwargsAuthFallback:
             from gateway.run import _resolve_runtime_agent_kwargs
             with pytest.raises(RuntimeError):
                 _resolve_runtime_agent_kwargs()
+
+    def test_legacy_fallback_is_appended_after_fallback_providers(self, tmp_path, monkeypatch):
+        """When both keys exist, the legacy entry still participates in resolution."""
+        config_path = tmp_path / "config.yaml"
+        config_path.write_text(
+            "fallback_providers:\n"
+            "  - provider: openrouter\n"
+            "    model: anthropic/claude-sonnet-4.6\n"
+            "fallback_model:\n"
+            "  provider: nous\n"
+            "  model: Hermes-4\n"
+        )
+
+        monkeypatch.setattr("gateway.run._hermes_home", tmp_path)
+
+        calls = []
+
+        def _mock_resolve(**kwargs):
+            requested = kwargs.get("requested")
+            calls.append(requested)
+            if requested == "openrouter":
+                raise RuntimeError("openrouter unavailable")
+            return {
+                "api_key": "nous-key",
+                "base_url": "https://portal.nousresearch.com/v1",
+                "provider": "nous",
+                "api_mode": "chat_completions",
+                "command": None,
+                "args": None,
+                "credential_pool": None,
+            }
+
+        with patch(
+            "hermes_cli.runtime_provider.resolve_runtime_provider",
+            side_effect=_mock_resolve,
+        ):
+            from gateway.run import _try_resolve_fallback_provider
+
+            result = _try_resolve_fallback_provider()
+
+        assert calls == ["openrouter", "nous"]
+        assert result["provider"] == "nous"
+        assert result["model"] == "Hermes-4"