test: handle model instances in discriminators (#8)

Pydantic invokes the Discriminator callable on both validate and
serialize paths. The two harness discriminators only inspected dicts,
so on dump they fell through to the first variant and emitted
PydanticSerializationUnexpectedValue warnings — ~25 per round-trip
pass. Adding isinstance arms before the dict check routes serialization
to the correct variant and silences the warnings, with the validate
path unchanged.

Author: chris-colinsky

tests/conformance/harness/expectations.py

@@ -224,7 +224,21 @@ def _discriminate_expected(
     blocks themselves don't know that, so we discriminate on the keys
     that ARE distinctive and fall back to graph-engine for plain
     ``final_state``-only fixtures.
+
+    Pydantic invokes this callable on both the validation and
+    serialization paths. On serialization the value is already one of the
+    concrete variants, so route by ``isinstance`` first; otherwise the
+    dump path falls through to ``graph_engine`` and warns that the
+    serialized variant doesn't match the union's first arm.
     """
+    if isinstance(value, GraphEngineExpected):
+        return "graph_engine"
+    if isinstance(value, LlmProviderExpected):
+        return "llm_provider"
+    if isinstance(value, PipelineUtilitiesExpected):
+        return "pipeline_utilities"
+    if isinstance(value, ObservabilityExpected):
+        return "observability"
     if not isinstance(value, dict):
         return "graph_engine"
     keys: set[str] = {str(k) for k in cast("dict[str, Any]", value)}

tests/conformance/harness/fixtures.py

@@ -241,7 +241,17 @@ def _discriminate_fixture(value: Any) -> Literal["llm_provider", "cases", "graph
     llm-provider fixtures (e.g. 003-message-validation) have BOTH —
     ``mock_provider`` is the load-bearing discriminator, ``cases`` is just
     the table style for sub-cases.
+
+    Also handle the serialization path (where the value is a concrete
+    variant) so a future ``model_dump`` through the top-level union
+    doesn't fall through to ``graph`` and warn.
     """
+    if isinstance(value, LlmProviderFixture):
+        return "llm_provider"
+    if isinstance(value, CasesFixture):
+        return "cases"
+    if isinstance(value, GraphFixture):
+        return "graph"
     if isinstance(value, dict):
         if "mock_provider" in value:
             return "llm_provider"