Clean up how we get beam-specific constants

> Could: This is messy to have here, it would be better to have some function like `get_beamline_constants` and that
> would give you an object like `I03Constants` for each beamline

_Originally posted by @DominicOram in https://github.com/DiamondLightSource/mx-bluesky/pull/1059#discussion_r2086962637_

Currently `DETECTOR_SIZE_PER_BEAMLINE` sits in `common/gridscan.py`. This is messy and we should create a function that gives us constants object for a certain beamline.