Make ModelCapabilities.from_dict defensive for missing fields

The runtime may return models (e.g. embedding models) where 'limits'
or 'supports' are absent. Default to empty objects instead of raising
ValueError, matching the lenient behavior in Go/C#/TypeScript SDKs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

python/copilot/client.py

@@ -389,13 +389,8 @@ def from_dict(obj: Any) -> ModelCapabilities:
         assert isinstance(obj, dict)
         supports_dict = obj.get("supports")
         limits_dict = obj.get("limits")
-        if supports_dict is None or limits_dict is None:
-            raise ValueError(
-                f"Missing required fields in ModelCapabilities: supports={supports_dict}, "
-                f"limits={limits_dict}"
-            )
-        supports = ModelSupports.from_dict(supports_dict)
-        limits = ModelLimits.from_dict(limits_dict)
+        supports = ModelSupports.from_dict(supports_dict) if supports_dict else ModelSupports()
+        limits = ModelLimits.from_dict(limits_dict) if limits_dict else ModelLimits()
         return ModelCapabilities(supports=supports, limits=limits)
 
     def to_dict(self) -> dict: