Move to `tombi` from `taplo`

Maybe we could replace it for something else altogether. Haven't tried, but tombi seems to be sued for the same purpose:

- https://reinout.vanrees.org/weblog/2026/03/18/tombi-precommit.html
- https://tombi-toml.github.io/tombi/

_Originally posted by @dalonsoa in https://github.com/ImperialCollegeLondon/python-template/pull/554#pullrequestreview-4289744569_

Start by locating the repository's existing taplo references and configuration, then read the linked tombi documentation to compare their supported purpose. Determine which template or project setup files would need the replacement and verify that the resulting configuration still performs the same checks. Done means taplo is replaced consistently and the relevant validation passes.

Written by the indexing model from the issue text.