feat(pay): 支持单参数 switchover 自定义键及通知回调空 appId 降级处理

[Copilot]

4.7.7.B 将 switchOver 改为必须传入 mchId + appId 两个参数，导致多租户场景（使用自定义唯一键管理配置）和通知回调（appId 可能为空）无法正常切换配置。

变更

新增单键 API（多租户场景）

• WxPayService.addConfig(String configKey, WxPayConfig) — 以任意唯一键注册配置
• WxPayService.removeConfig(String configKey) — 以任意唯一键移除配置
• 已有的 switchover(String) / switchoverTo(String) 本身已支持精确 key 匹配，Javadoc 补充说明其可接受任意自定义键

修复通知回调空 appId 问题

• switchover(mchId, appId) / switchoverTo(mchId, appId) 当 appId 为 null 或空白时，自动降级为 switchover(mchId) 而非以 mchId_null 查找失败

文档

• MULTI_APPID_USAGE.md 补充多租户用法示例及完整 API 对比表格

使用示例

// 多租户场景：使用租户 ID 作为配置键
payService.addConfig("tenant_001", tenant1Config);
payService.addConfig("tenant_002", tenant2Config);

// 单参数切换，不依赖 mchId/appId 格式
payService.switchover("tenant_001");

// 链式调用
payService.switchoverTo("tenant_001").unifiedOrder(request);

// 通知回调中 appId 为空时可正常切换
payService.switchover(notifyResult.getMchId(), notifyResult.getAppid()); // appId 为 null 时自动降级

Original prompt

---

This section details on the original issue you should resolve

<issue_title>关于switchOver改成mchId+appId的入参的咨询</issue_title>
<issue_description>最近试用了一下4.7.7.B发现调用switchOver改成了mchId+appId的入参，感觉原来使用一个id作为入参的方式挺好的。
1.通过一个全局唯一id获取租户的配置感觉使用更方便一些
2.也不是所有的服务都依赖appid这个参数，比如微信支付通知回调
希望能保留或者兼容原来单参数switchOver的方法</issue_description>

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

• Fixes 关于switchOver改成mchId+appId的入参的咨询 #3661

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[Copilot]

Pull request overview

This PR addresses a regression introduced in v4.7.7.B, where switchOver was changed to require both mchId + appId parameters. This broke multi-tenant (SaaS) scenarios (using custom unique keys for config management) and payment notification callbacks (where appId may be absent). The fix adds a single-key API overload and adds a graceful fallback when appId is blank.

Changes:

• Added addConfig(String configKey, WxPayConfig) and removeConfig(String configKey) to WxPayService interface and implemented in BaseWxPayServiceImpl, allowing any custom string (e.g., a tenant ID) as the config key.
• Modified switchover(mchId, appId) and switchoverTo(mchId, appId) to automatically fall back to the single-argument form when appId is blank/null, fixing notification callback failures.
• Updated MULTI_APPID_USAGE.md with new multi-tenant usage examples and an API comparison table; added corresponding tests in MultiAppIdSwitchoverTest.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
weixin-java-pay/src/main/java/com/github/binarywang/wxpay/service/WxPayService.java	Adds addConfig(String, WxPayConfig) and removeConfig(String) interface methods; updates Javadoc for single-arg switchover/switchoverTo to describe dual-mode matching
weixin-java-pay/src/main/java/com/github/binarywang/wxpay/service/impl/BaseWxPayServiceImpl.java	Implements the new single-key addConfig/removeConfig methods; adds null/blank appId fallback in switchover(mchId, appId) and switchoverTo(mchId, appId)
weixin-java-pay/src/test/java/com/github/binarywang/wxpay/service/impl/MultiAppIdSwitchoverTest.java	Adds test cases for custom-key add/remove/switchover and for null/blank appId fallback behavior
weixin-java-pay/MULTI_APPID_USAGE.md	Adds new section 2 for multi-tenant custom-key usage, updates API table, adds description of null-appId fallback; contains duplicate section numbering

---

The only identified issue is a documentation inconsistency: the new section ### 2. 使用自定义唯一键配置（多租户场景） (line 62) was inserted without renumbering the subsequent ### 2. 获取配置的方式 (line 102) through ### 5. sections, resulting in two sections both numbered "2.". The code changes themselves are correct, consistent, and well-tested.