Use PromptKit prepared execution for structured model calls

internal/framework/structuredoutput/failure.go

@@ -0,0 +1,27 @@
+package structuredoutput
+
+import "errors"
+
+// FailureKind separates invalid generated content from operational failures.
+type FailureKind string
+
+const (
+	FailureMalformedOutput FailureKind = "malformed_output"
+	FailureRuntime         FailureKind = "runtime"
+)
+
+// Failure exposes a safe message while retaining the typed cause for callers.
+// The cause must not be serialized or formatted directly in user artifacts.
+type Failure struct {
+	Kind    FailureKind
+	Message string
+	Cause   error
+}
+
+func (f *Failure) Error() string { return f.Message }
+func (f *Failure) Unwrap() error { return f.Cause }
+
+func IsMalformedError(err error) bool {
+	var failure *Failure
+	return errors.As(err, &failure) && failure.Kind == FailureMalformedOutput
+}