API view: FATAL diagnostics issues for Java APIs

This issue is to track the list of issues we want to mark as FATAL for Java APIs.

- [ ] Missing module-info.java
- [ ] Missing javadocs for public APIs
- [ ] Exposing implementation types in public APIs

Start at the API view and trace how Java API diagnostics are defined and classified. Review the three listed conditions—missing module-info.java, missing public-API javadocs, and exposed implementation types—and identify the relevant tests or entry points. Done means each intended condition is reported as FATAL for Java APIs.

Written by the indexing model from the issue text.