[rest.li] Fix race condition in FileStore.get() causing IndexOutOfBoundsException

[emilyki]

Summary

• Replace manual file.length() + FileInputStream read loop with Files.readAllBytes() in FileStore.get() to eliminate a TOCTOU race condition where concurrent file modification (e.g., ZooKeeper property sync) causes IndexOutOfBoundsException
• Add RuntimeException catch in StoreEventPublisher.startPublishing() so unexpected errors still publish a null value instead of propagating uncaught and preventing D2 service discovery initialization

Testing Done

• Existing FileStoreTest tests pass
• Verified compilation with ./gradlew :d2:compileJava

[jcleezer]

Looks good!