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
+}