Guard subset filter against non-mapping expected_usage

Address PR review feedback: the conformance fixture's typed model
allows usage: null. When that happens, expected_usage is None and
the subset filter's k in expected_usage raises TypeError instead of
producing a clean assertion failure.

Wrap the subset filter in an isinstance(dict) guard. The mapping path
keeps the same subset semantics; the non-mapping path falls back to
direct comparison so the assert fires with a clear shape mismatch.

Author: chris-colinsky

tests/conformance/test_llm_provider.py

@@ -430,17 +430,24 @@ def _assert_response_matches(actual: Response, expected: Mapping[str, Any]) -> N
         assert actual.finish_reason == expected["finish_reason"]
     if "usage" in expected:
         expected_usage = expected["usage"]
-        # Subset comparison: spec fixtures assert which usage fields
-        # MUST be present with what values. Impl-extension fields
-        # outside the fixture's expected set (e.g., the 0047
-        # cache-stat fields on impls that have adopted them but
-        # against fixtures that pre-date the proposal) are ignored
-        # when the fixture doesn't assert about them. A fixture key
-        # that's absent from actual surfaces as a missing key in the
-        # filtered dict and fails the comparison; the impl can't
-        # silently drop a field the spec requires.
         actual_usage_full = actual.usage.model_dump()
-        actual_usage = {k: v for k, v in actual_usage_full.items() if k in expected_usage}
+        # Subset comparison when the fixture asserts about specific
+        # usage fields: spec fixtures pin which fields MUST be present
+        # with what values. Impl-extension fields outside the fixture's
+        # expected set (e.g., the 0047 cache-stat fields on impls that
+        # have adopted them but against fixtures that pre-date the
+        # proposal) are ignored when the fixture doesn't assert about
+        # them. A fixture key that's absent from actual surfaces as a
+        # missing key in the filtered dict and fails the comparison;
+        # the impl can't silently drop a field the spec requires.
+        #
+        # Non-mapping expected_usage (e.g., a fixture sets usage: null)
+        # falls back to direct comparison so the assertion fires with a
+        # clean shape mismatch rather than crashing on the subset filter.
+        if isinstance(expected_usage, dict):
+            actual_usage = {k: v for k, v in actual_usage_full.items() if k in expected_usage}
+        else:
+            actual_usage = actual_usage_full
         assert actual_usage == expected_usage, (
             f"usage mismatch: actual={actual_usage}, expected={expected_usage}"
         )