fix(java): 部署级字段从全局配置读取，不再依赖线程本地 MDC

[wyhhai]

缺陷

Java SDK 输出的结构化日志中，契约要求的固定标签 service / env / instance 整条缺失（om-webserver 接入实测）。

实际输出：
{"time":"...","level":"info","msg":"...","community":"unknown","request_id":"...","logger":"LogUtil.java:107"}

根因

ObsLogging.init() 把部署级字段写入 SLF4J MDC（线程本地存储）：

• init() 在应用启动线程执行，只有该线程的 MDC 有这四个值
• 业务请求运行在 web 容器工作线程，其 MDC 仅含 enrich() 写入的 community / request_id
• ObsJsonProvider.writeTo() 从事件线程的 MDC 取这些键，取不到即省略

部署级字段本质是全局静态配置（来源 OBS_* 环境变量），用线程本地 MDC 承载属于错误建模。

修复

1. ObsLogging 增加公开静态访问器 deploymentService() / deploymentEnv() / deploymentInstance()，
   deploymentCommunity() 改为公开；init() 不再写 MDC，未 init 时回退契约默认 Env.DEFAULT_VALUE
2. ObsJsonProvider.writeTo()：
   • service / env / instance：直接从全局配置读取（不依赖线程 MDC，恒非空）
   • community：保持「MDC 请求级覆盖 > 部署默认」语义不变（恒非空）
   • request_id / trace_id / span_id：仍走 MDC，空值省略
3. 补回归测试：模拟请求线程（新线程）打日志，验证部署字段完整输出；未 init 时访问器回退契约默认

验证

• mvn test：42 个测试全部通过（新增 4 个）
• 覆盖：ObsJsonProviderTest +2（请求线程模拟、无请求级 MDC 回退默认）；ObsLoggingTest +2（访问器取值、未 init 回退 unknown）

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