[Controller] Migrate GET_BACKUP_VERSION endpoint to gRPC

[pthirun]

Summary

• Migrate the GET_BACKUP_VERSION HTTP endpoint to gRPC
• Add GetBackupVersionGrpcRequest and GetBackupVersionGrpcResponse proto messages
• Add getBackupVersion RPC to VeniceControllerGrpcService
• Refactor StoresRoutes.getBackupVersion() to use the shared request handler
• Add VeniceNoStoreException handling in ControllerGrpcServerUtils for NOT_FOUND status
• Add comprehensive tests for the handler and gRPC service

Test plan

• Unit tests for VeniceControllerRequestHandler.getBackupVersion() (parent and child controller scenarios)
• Unit tests for VeniceControllerGrpcServiceImpl.getBackupVersion() (success and error cases)
• Unit tests for StoresRoutes.getBackupVersion() (HTTP route integration)
• All existing tests pass

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