add unit tests for artifact mixin

tests/unit/artifacts/test_artifact_mixin.py

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+
+from dataclasses import dataclass
+import pytest
+
+
+from dbt.artifacts.schemas.base import (
+    ArtifactMixin,
+    BaseArtifactMetadata,
+    schema_version,
+)
+
+@dataclass
+@schema_version("manifest", 12)
+class Artifact(ArtifactMixin):
+    a: int
+
+
+@dataclass
+@schema_version("manifest", 12)
+class ArtifactMinorSchemaChange(ArtifactMixin):
+    a: int
+    b: int = 2
+
+
+class TestMinorSchemaChange:
+    @pytest.fixture
+    def artifact(self):
+        return Artifact(a = 1, metadata=BaseArtifactMetadata(dbt_schema_version="1.9.0"))
+
+    @pytest.fixture
+    def artifact_minor_schema_change(self):
+        return ArtifactMinorSchemaChange(a=1, metadata=BaseArtifactMetadata(dbt_schema_version="1.9.0"))
+
+    def test_serializing_new_default_field_is_backward_compatabile(
+        self, artifact_minor_schema_change
+    ):
+        # old code (using old class) can create an instance of itself given new data (new class)
+        new_data = artifact_minor_schema_change.to_dict()
+        # Ensure new data has additional field
+        assert new_data["b"] == 2
+        artifact = Artifact.from_dict(new_data)
+        
+        assert artifact.a == 1
+        # Additional fields are ignored
+        assert not hasattr(artifact, "b")
+
+    def test_serializing_new_default_field_is_forward_compatible(self, artifact):
+        # new code (using new class) can create an instance of itself given old data (old class)
+        artifact = ArtifactMinorSchemaChange.from_dict(artifact.to_dict())
+
+        assert artifact.a == 1
+        assert artifact.b == 2
+
+    def test_serializing_removed_default_field_is_backward_compatabile(self, artifact):
+        # old code (using old class with default field) can create an instance of itself given new data (class w/o default field)
+        old_artifact = ArtifactMinorSchemaChange.from_dict(artifact.to_dict())
+        
+        # set to the default value when not provided in data
+        assert old_artifact.b == 2
+
+    def test_serializing_removed_default_field_is_forward_compatible(
+        self, artifact_minor_schema_change
+    ):
+        # new code (using class without default field) can create an instance of itself given old data (class with old field)
+        Artifact.from_dict(artifact_minor_schema_change.to_dict())