Every second collection on i15-1 fails

[DominicOram]

During testing on i15-1 we found that every second eiger collection is failing when we trigger "arm_when_ready" with the following error:

> File "/app/.venv/lib/python3.11/site-packages/fastcs/methods/command.py", line 42, in command
    return await self._fn()
                 │    └ <bound method EigerOdinController.arm_when_ready of EigerOdinController(path=None, sub_controllers=['OD', 'detector', 'stream...
                 └ <fastcs.methods.command.Command object at 0x7f1ba073dad0>
  File "/app/.venv/lib/python3.11/site-packages/fastcs_eiger/controllers/odin/eiger_odin_controller.py", line 52, in arm_when_ready
    raise TimeoutError("Eiger fan not ready") from e
TimeoutError: Eiger fan not ready
[2026-06-24 11:45:40.063+0000 E] Exception when running dispatched callback                                                       root [/app/.venv/lib/python3.11/site-packages/fastcs/logging/__init__.py:86]

We could not reliably reproduce the issue manually through the phoebus screen but it consistently happens every second collection when triggered through ophyd-async.

Acceptance Criteria

• We can reliably trigger arm the detector when running through ophyd-async