Refactor OPM/Neuromag grouping

[PragnyaKhandelwal]

Reference issue (if any)

Follow‑up to #13866.

What does this implement/fix?

• Makes tangential OPM topomaps use unsigned RMS magnitude (aligned with Neuromag grads).
• Uses per‑group vlim and non‑diverging colormap for tangential maps.
• Adds a shared helper to unify grouping/merging logic for OPM and Neuromag‑style channels.
• Updates OPM grouped topomap tests for new axes/colorbar behavior.

Additional information

• Draft PR; please review after ENH: group triaxial OPM topomaps by orientation #13866 merges (I will rebase then).

[larsoner]

Failures look related. The ubuntu-latest / pip job runs the most complete set of tests, so you can look here for a good list of failures:

https://github.com/mne-tools/mne-python/actions/runs/26439596935/job/77830628215?pr=13921

Once you have these working locally feel free to push fixes!