Centralize validator classification and malformed output handling

internal/framework/structuredoutput/malformed.go

@@ -0,0 +1,28 @@
+package structuredoutput
+
+import "strings"
+
+var malformedMarkers = []string{
+	"malformed structured output",
+	"decode structured output:",
+	"decode provider response envelope:",
+	"provider response missing choices",
+	"provider response missing assistant message content",
+	"provider response assistant message content is empty",
+	"provider response assistant message content is not valid JSON",
+}
+
+// IsMalformedError reports whether err matches provider malformed
+// structured-output failure markers that should be downgraded.
+func IsMalformedError(err error) bool {
+	if err == nil {
+		return false
+	}
+	msg := err.Error()
+	for _, marker := range malformedMarkers {
+		if strings.Contains(msg, marker) {
+			return true
+		}
+	}
+	return false
+}

internal/framework/structuredoutput/malformed_test.go

@@ -0,0 +1,32 @@
+package structuredoutput
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestIsMalformedError(t *testing.T) {
+	cases := []struct {
+		name string
+		err  error
+		want bool
+	}{
+		{name: "nil", err: nil, want: false},
+		{name: "generic", err: errors.New("network timeout"), want: false},
+		{name: "malformed", err: errors.New("malformed structured output"), want: true},
+		{name: "decode structured", err: errors.New("decode structured output: unexpected end of JSON input"), want: true},
+		{name: "missing choices", err: errors.New("provider response missing choices"), want: true},
+		{name: "missing content", err: errors.New("provider response missing assistant message content"), want: true},
+		{name: "empty content", err: errors.New("provider response assistant message content is empty"), want: true},
+		{name: "invalid content json", err: errors.New("provider response assistant message content is not valid JSON"), want: true},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := IsMalformedError(tc.err)
+			if got != tc.want {
+				t.Fatalf("IsMalformedError(%v): got=%v want=%v", tc.err, got, tc.want)
+			}
+		})
+	}
+}