feat(captcha): LLM text-image OCR solver with strict-retry

internal/captcha/json.go

@@ -0,0 +1,13 @@
+package captcha
+
+import "strings"
+
+// extractJSON returns the first {...} JSON object found in s, or s unchanged.
+func extractJSON(s string) string {
+	start := strings.Index(s, "{")
+	end := strings.LastIndex(s, "}")
+	if start >= 0 && end > start {
+		return s[start : end+1]
+	}
+	return s
+}