Align execution errors with the shared catalog and add optional logs.

Contract codes now use the agent.* values, and runtime can emit structured logs without changing results when no logger is configured.

Co-authored-by: Cursor <cursoragent@cursor.com>

contract/contract_test.go

@@ -26,7 +26,7 @@ func TestFakeBackendSuite(t *testing.T) {
 func TestFakeCancelSemantics(t *testing.T) {
 	block := make(chan struct{})
 	b := &conformance.FakeBackend{
-		Desc: contract.BackendDescriptor{Name: "fake", Capabilities: []contract.Capability{contract.CapCancel}},
+		Desc:       contract.BackendDescriptor{Name: "fake", Capabilities: []contract.Capability{contract.CapCancel}},
 		BlockUntil: block,
 	}
 	conformance.Run(t, conformance.Case{
@@ -41,3 +41,23 @@ func TestFakeCancelSemantics(t *testing.T) {
 		t.Fatal("backend must observe ctx cancellation before returning")
 	}
 }
+
+func TestErrorDuckTypeForLogs(t *testing.T) {
+	e := &contract.Error{
+		Code:        contract.ErrProviderUnavailable,
+		Reason:      "down",
+		Retryable:   true,
+		SideEffects: contract.SideEffectNone,
+	}
+	if e.ErrorCode() != "agent.provider_unavailable" {
+		t.Fatal(e.ErrorCode())
+	}
+	fields := e.ErrorFields()
+	if fields["reason"] != "down" || fields["side_effects"] != "none" || fields["retryable"] != true {
+		t.Fatalf("%v", fields)
+	}
+	var nilErr *contract.Error
+	if nilErr.ErrorCode() != "" || nilErr.ErrorFields() != nil {
+		t.Fatal("nil error")
+	}
+}