fix(test): check error return value in protobuf marshaling test

Fix errcheck linter error by properly handling return values in the
panic test assertion.

Co-Authored-By: Claude <[email protected]>

Author: jra3

internal/hardware/graph/error_paths_test.go

@@ -365,13 +365,10 @@ func TestProtobufMarshaling_Errors(t *testing.T) {
 		// The actual functions validate data before marshaling, so we're
 		// testing that the error is properly propagated
 
-		_, err := builder.createContainsRelationship(nil, nil, "physical")
-		// This should handle nil refs gracefully or return an error
-		// (depending on implementation - anypb.New will handle the proto message)
+		// Test that creating relationships with nil refs doesn't panic
 		assert.NotPanics(t, func() {
-			builder.createContainsRelationship(nil, nil, "physical")
+			_, _ = builder.createContainsRelationship(nil, nil, "physical")
 		})
-		_ = err // Error handling is implementation-specific
 	})
 }