Fix Java annotation processor support to actually run processors

[oyvindberg]

Summary

This PR fixes Java annotation processor support in bleep. The previous implementation only added the -s option for generated sources but didn't actually enable annotation processing - Bloop was adding -proc:none which disabled it.

Changes

• Removed the simple AnnotationProcessing(enabled: Boolean) model
• Added a new AnnotationProcessor model with required className field (extensible for future features)
• Updated Java model to use JsonSet[AnnotationProcessor] instead of Option[AnnotationProcessing]
• Modified GenBloopFiles to pass explicit -processor options to javac with comma-separated processor class names
• Only adds -proc:none when no processors are configured
• Updated tests to verify -processor option is correctly generated
• Added documentation to CLAUDE.md

Breaking Change

The old annotationProcessing.enabled field is replaced with an explicit list of annotation processors:

Before:

java:
  annotationProcessing:
    enabled: true

After:

java:
  annotationProcessors:
    - className: lombok.launch.AnnotationProcessorHider$AnnotationProcessor
    - className: com.google.auto.value.processor.AutoValueProcessor

Test plan

• All existing tests pass
• Added test assertions to verify -processor option is generated
• Tested with Lombok annotation processor in integration test

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