ContentBlockManager already declares task_arg_buffer, task_args_emitted,
tool_ids. Remove defensive getattr/isinstance checks from _process_tool_call
and _flush_task_arg_buffers.
Update test_subagent_interception to set task_arg_buffer, task_args_emitted,
tool_ids on mock so it behaves like real ContentBlockManager.
Co-authored-by: Ali Khokhar <alishahryar2@gmail.com>