Implements JSON format support for WeChat Mini Program outbound messages by adding the WxMaJsonOutMessage class.

Problem

Currently, the WeChat Mini Program SDK only supports XML format for message push responses through WxMaXmlOutMessage. According to WeChat's official documentation, JSON format should also be supported for message push scenarios.

Solution

Added WxMaJsonOutMessage class that provides JSON format support while maintaining full compatibility with existing XML functionality:

// Create JSON format message
WxMaJsonOutMessage jsonMessage = WxMaJsonOutMessage.builder()
 .fromUserName("your_mini_program")
 .toUserName("user_openid")
 .msgType(WxConsts.XmlMsgType.TEXT)
 .createTime(System.currentTimeMillis() / 1000)
 .build();
// Convert to JSON
String jsonResponse = jsonMessage.toJson();
// Output: {"toUserName":"user_openid","fromUserName":"your_mini_program","createTime":1758727378,"msgType":"text"}
// Encrypted JSON support
String encryptedJson = jsonMessage.toEncryptedJson(config);

Key Features

• JSON Serialization: Uses existing WxMaGsonBuilder for consistent JSON formatting
• Encryption Support: Includes toEncryptedJson(WxMaConfig) method for encrypted responses
• Builder Pattern: Follows the same builder pattern as WxMaXmlOutMessage
• Zero Breaking Changes: Existing XML functionality remains unchanged
• Consistent API: Mirrors the interface of WxMaXmlOutMessage for familiar usage

Implementation Details

The new class follows established patterns from other WxJava modules (like weixin-java-cp's WxCpMessage.toJson()) and maintains the project's coding standards:

• Uses Lombok annotations for consistency
• Includes comprehensive unit tests
• Passes all checkstyle validations
• Follows the existing serialization patterns

This minimal implementation provides developers the flexibility to choose between XML and JSON formats based on their specific needs while keeping the codebase clean and maintainable.

Fixes #小程序消息推送没有对消息体是json数据格式支持

Fixes #3376

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