fix(model_metadata): never fuzzy-match an empty model name against endpoint catalogs

'' is a substring of every catalog key, so _resolve_endpoint_context_length
with an empty model name "matched" whatever the endpoint listed first —
on the Nous portal that is currently a 32K embedding model, which poisoned
the resolved context length and made AIAgent init fail the 64K minimum.
This is what turned tests/run_agent/test_primary_runtime_restore.py::
TestTryRecoverPrimaryTransport::test_allowed_for_nous_anthropic_messages
red on every PR (CI slice 7/12) after the portal catalog reordered.

Single-model endpoints still resolve with an empty name (unambiguous);
non-empty names keep the substring fuzzy match.

agent/model_metadata.py

@@ -1399,7 +1399,12 @@ def _resolve_endpoint_context_length(
     if not matched:
         if len(endpoint_metadata) == 1:
             matched = next(iter(endpoint_metadata.values()))
-        else:
+        elif model:
+            # Substring fuzzy match — only meaningful with a non-empty model
+            # name.  An empty string is a substring of EVERY key, which would
+            # "match" whatever model the endpoint happens to list first (e.g.
+            # a 32K embedding model on the Nous portal) and poison the
+            # resolved context length for the whole agent.
             for key, entry in endpoint_metadata.items():
                 if model in key or key in model:
                     matched = entry

tests/agent/test_model_metadata.py

@@ -591,6 +591,30 @@ class TestNousPortalContextResolution:
 
 
 
+    @patch("agent.model_metadata.fetch_endpoint_model_metadata")
+    def test_empty_model_never_fuzzy_matches_endpoint_catalog(self, mock_fetch):
+        """An empty model name must not substring-match arbitrary catalog
+        entries — '' is a substring of every key, so pre-fix it "matched"
+        whatever the endpoint listed first (e.g. a 32K embedding model on
+        the Nous portal) and poisoned the resolved context length."""
+        import agent.model_metadata as mm
+        mock_fetch.return_value = {
+            "voyageai/voyage-code-4": {"context_length": 32_000},
+            "x-ai/grok-4.6": {"context_length": 500_000},
+        }
+        assert mm._resolve_endpoint_context_length(
+            "", "https://inference-api.nousresearch.com/v1"
+        ) is None
+        # Non-empty names still fuzzy-match.
+        assert mm._resolve_endpoint_context_length(
+            "grok-4.6", "https://inference-api.nousresearch.com/v1"
+        ) == 500_000
+        # Single-model endpoints still resolve even with an empty name.
+        mock_fetch.return_value = {"only-model": {"context_length": 131_072}}
+        assert mm._resolve_endpoint_context_length(
+            "", "http://localhost:8080/v1"
+        ) == 131_072
+
     @patch("agent.model_metadata.fetch_endpoint_model_metadata")
     @patch("agent.model_metadata.fetch_model_metadata")
     def test_openrouter_fallback_is_not_persisted(