Use PromptKit prepared execution for structured model calls

internal/framework/structuredoutput/failure_test.go

@@ -0,0 +1,18 @@
+package structuredoutput
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestFailureClassificationKeepsCauseAndSafeMessage(t *testing.T) {
+	cause := errors.New("provider token secret-value")
+	malformed := &Failure{Kind: FailureMalformedOutput, Message: "malformed structured output", Cause: cause}
+	if !IsMalformedError(malformed) || !errors.Is(malformed, cause) || malformed.Error() != "malformed structured output" {
+		t.Fatalf("malformed classification: %v", malformed)
+	}
+	runtime := &Failure{Kind: FailureRuntime, Message: "structured model call failed", Cause: cause}
+	if IsMalformedError(runtime) || IsMalformedError(cause) || IsMalformedError(nil) || !errors.Is(runtime, cause) || runtime.Error() != "structured model call failed" {
+		t.Fatalf("runtime classification: %v", runtime)
+	}
+}