[Tracking] Separate Java build and test in CI

[rhdong]

Summary
Currently, ci/test_java.sh runs both build and test via ./build.sh java. This issue tracks the plan to separate them after the Java CI becomes stable, so that Java tests in CI assume prebuilt artifacts are available and only run tests.

Motivation

• Avoid rebuilding Java bindings in every CI run
• Allow for future use of prebuilt or published Java packages
• Simplify test CI jobs

Planned Work

• Ensure Java artifacts are prebuilt and made available to the test job (via package download)
• Remove ./build.sh java from ci/test_java.sh

Context
This change prepares the path for possibly distributing cuvs-java packages later, and was suggested during review in PR #805.