docs: clarify that micropay sign/appid/nonceStr are auto-filled by the SDK

[Copilot]

Users calling wxPayService.micropay() were getting "签名错误" after manually computing and setting sign via SignUtils — which conflicts with the SDK's own signing logic.

Changes

• Extended the WxPayController#micropay Javadoc to explicitly state that appid, mch_id, nonce_str, and sign are populated automatically from merchant config; manually setting them (especially sign) causes signature failures
• Added a minimal usage example showing only the fields callers need to set:

WxPayMicropayRequest request = new WxPayMicropayRequest();
request.setBody("商品描述");
request.setOutTradeNo("商户系统内部订单号");
request.setTotalFee(100);          // 单位：分
request.setSpbillCreateIp("终端IP");
request.setAuthCode("用户付款码");
WxPayMicropayResult result = wxPayService.micropay(request);

Original prompt

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This section details on the original issue you should resolve

<issue_title>付款码支付 WxPayMicropayRequest 中的签名,怎么弄的?</issue_title>
<issue_description>How do I get a payment code to pay a signature in WxPayMicropayRequest?

Version 4.8.0

String nonceStr = IdUtil.fastSimpleUUID();

WxPayMicropayRequest request = new WxPayMicropayRequest();
request.setAppid(wxPayService.getConfig().getAppId());
request.setMchId(wxPayService.getConfig().getMchId());
request.setNonceStr(nonceStr);
request.setBody(body);
request.setOutTradeNo(outTradeNo);
request.setTotalFee(totalFee);
request.setSpbillCreateIp(ip);
request.setAuthCode(authCode);


HashMap<String,String> map = new HashMap<>();
map.put("appid",wxPayService.getConfig().getAppId());
map.put("mch_id",wxPayService.getConfig().getMchId());
map.put("nonce_str",nonceStr);
map.put("body",body);
map.put("out_trade_no",outTradeNo);
map.put("total_fee",totalFee.toString());
map.put("spbill_create_ip",ip);
map.put("auth_code",authCode);

String[] ignoredParams = new String[]{};
String sign = SignUtils.createSign(map, "", apiV2Key, ignoredParams);

request.setSign(sign); 

WxPayMicropayResult micropayResult = wxPayService.micropay(request);

----报错-----

返回代码：[FAIL]，返回信息：[签名错误，请检查后再试]，微信返回的原始报文：
<return_code></return_code>
<return_msg></return_msg>

@binarywang 签名部分能给个例子吗?谢谢🙏</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 付款码支付 WxPayMicropayRequest 中的签名,怎么弄的? #151

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[Copilot]

Pull request overview

This PR clarifies the micropay API usage in the demo controller docs to prevent callers from manually setting SDK-managed common fields (especially sign), which can lead to signature failures.

Changes:

• Expanded WxPayController#micropay Javadoc to state appid/mch_id/nonce_str/sign are auto-filled and signed by the SDK.
• Added a minimal WxPayMicropayRequest usage example showing only required business fields.

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