refactor(go/core): omit empty details in ToReflectionError JSON output

Only allocate ReflectionErrorDetails when stack or traceId is present,
leveraging the omitempty tag to produce cleaner JSON output.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Zereker

go/core/error.go

@@ -107,13 +107,19 @@ func (e *GenkitError) Unwrap() error {
 
 // ToReflectionError returns a JSON-serializable representation for reflection API responses.
 func (e *GenkitError) ToReflectionError() ReflectionError {
-	errDetails := &ReflectionErrorDetails{}
+	var errDetails *ReflectionErrorDetails
 	if e.Details != nil {
-		if stackVal, ok := e.Details["stack"].(string); ok {
-			errDetails.Stack = &stackVal
-		}
-		if traceVal, ok := e.Details["traceId"].(string); ok {
-			errDetails.TraceID = &traceVal
+		stackVal, stackOk := e.Details["stack"].(string)
+		traceVal, traceOk := e.Details["traceId"].(string)
+
+		if stackOk || traceOk {
+			errDetails = &ReflectionErrorDetails{}
+			if stackOk {
+				errDetails.Stack = &stackVal
+			}
+			if traceOk {
+				errDetails.TraceID = &traceVal
+			}
 		}
 	}
 	return ReflectionError{

go/core/error_test.go

@@ -157,8 +157,8 @@ func TestGenkitErrorToReflectionError(t *testing.T) {
 		if re.Message != "success" {
 			t.Errorf("Message = %q, want %q", re.Message, "success")
 		}
-		if re.Details.Stack != nil {
-			t.Error("expected nil stack")
+		if re.Details != nil {
+			t.Error("expected nil details")
 		}
 	})
 }