build: align 3.0.x pom.xml with okhttp3-extension template (JDK 21)

- Restructure pom.xml to match okhttp3-extension feature/3.0.x template
- Adopt Aliyun distributionManagement, central-publishing-maven-plugin
- Use okhttp 5.4.0 (JDK 21+ line), jackson 2.22.1, slf4j 2.1.0
- Use Lombok 1.18.46, commons-exec 1.4.0, junit 5.11.4
- Add maven.compiler.release for JDK 21 modular compile
- Adopt full pluginManagement: enforcer, compiler, deploy, gpg, install,
  jar, javadoc, release, resources, source, surefire, central-publishing
- Use surefire 3.5.6 (3.6.0 not yet released; 3.5.6 is current stable)
- Add OpenCodeRequestContext (workspace routing context)
- Add OpenCodeHttpClientTest (MockWebServer integration test)
- Cherry-pick 2.7.x OpenCodeClient/HttpClient/SseClient to accept
  OpenCodeRequestContext for workspace routing

Author: loong10k