Feat/add publish sdk gh action

[pascalwilbrink]

Summary

Adds .github/workflows/publish-java-sdk.yml, a manually triggered workflow for
releasing the community Java SDK (sdks/community/java/ag-ui) to Maven Central.

The run is split into two jobs:

1. build-and-test — checks out, sets up JDK 17 (Temurin, Maven cache) and
   runs mvn install for the selected reactor.
2. publish — runs only after the tests pass, in the maven environment, and
   deploys with -Prelease deploy -DskipTests.

[pascalwilbrink]

This publishes the ag-ui sdks to maven central. This is needed, because the framework integrations depend on this, rather than a locally resolved dependency. This is done, so all changes to the core sdk need to be officially published first