build: align 1.0.x pom.xml with okhttp3-extension template (JDK 1.8)

- Restructure pom.xml to match okhttp3-extension feature/1.0.x template
- Adopt Aliyun distributionManagement, central-publishing-maven-plugin
- Use okhttp 4.12.0 (last JDK 8 compatible), jackson 2.18.9, slf4j 2.0.18
- Use Lombok 1.18.46, commons-exec 1.4.0, junit 5.11.4
- Adopt full pluginManagement: enforcer, compiler, deploy, gpg, install,
  jar, javadoc, release, resources, source, surefire, central-publishing
- Add OpenCodeRequestContext (workspace routing context)
- Add OpenCodeHttpClientTest (MockWebServer integration test)

Author: loong10k