Fix payment callback parsing error with helpful format detection

[Copilot]

The parseOrderNotifyResult method was failing with a cryptic XmlPullParserException when developers passed JSON data from WeChat Pay V3 API instead of XML data expected by the V2 API method.

Problem: When using Spring Boot 3.4.2 with WxJava 4.7.7.B, calling parseOrderNotifyResult with V3 JSON notification data resulted in:

XmlPullParserException: only whitespace content allowed before start tag and not { (position: START_DOCUMENT seen {... @1:2)

This error occurs because:

• V3 API sends notifications in JSON format with structure like {"id": "...", "resource_type": "encrypt-resource", ...}
• V2 API expects XML format like <xml><appid>...</appid>...</xml>
• The method attempts to parse JSON as XML, causing the XStream parser to fail

Solution: Added format detection logic to parseOrderNotifyResult method that:

1. Detects when input data starts with { (JSON format)
2. Throws a helpful WxPayException with clear guidance:

   "检测到V3版本的JSON格式通知数据，请使用parseOrderNotifyV3Result方法解析。V3 API需要传入SignatureHeader参数进行签名验证。"

3. Maintains full backward compatibility for XML data

Usage Example:

// Before: Cryptic XmlPullParserException
// After: Clear guidance message
public Result notify(@RequestBody String notifyData) {
    try {
        // This now provides helpful error for JSON data
        final WxPayOrderNotifyResult notifyResult = wxPayService.parseOrderNotifyResult(notifyData);
    } catch (WxPayException e) {
        // Clear message directing to use parseOrderNotifyV3Result for V3 API
    }
}

// Correct usage for V3 JSON notifications:
public Result notifyV3(@RequestBody String notifyData, HttpServletRequest request) {
    SignatureHeader header = new SignatureHeader(
        request.getHeader("Wechatpay-Timestamp"),
        request.getHeader("Wechatpay-Nonce"), 
        request.getHeader("Wechatpay-Signature"),
        request.getHeader("Wechatpay-Serial")
    );
    final WxPayNotifyV3Result notifyResult = wxPayService.parseOrderNotifyV3Result(notifyData, header);
}

This change provides immediate value to developers migrating from V2 to V3 WeChat Pay APIs by replacing confusing parser errors with actionable guidance.

Fixes #3699.

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