fix(agent): handle YAML null value in context_length_cache

_load_context_cache() returned None when context_length_cache.yaml
contained 'context_lengths:' (no value) — YAML parses this as
{'context_lengths': None} and dict.get(key, default) only returns
the default when the key is absent, not when the value is None.

This caused AttributeError in every downstream caller (issue #47135).

Fix: use 'or {}' instead of default= so both absent key and
None value return an empty dict.

Fixes #47135

agent/model_metadata.py

@@ -1077,7 +1077,7 @@ def _load_context_cache() -> Dict[str, int]:
     try:
         with open(path, encoding="utf-8") as f:
             data = yaml.safe_load(f) or {}
-        return data.get("context_lengths", {})
+        return data.get("context_lengths") or {}
     except Exception as e:
         logger.debug("Failed to load context length cache: %s", e)
         return {}