Hotfix/from mf6

[p-ortega]

Summary

External-file mode had quietly stopped working when set with set_config(reactive_externalio=True). That form set one copy of the flag while the code read another, so the run skipped writing its chemistry data and then crashed with "initial condition not found". The nested form set_config(reactive={"externalio": True}) always worked, and this fix makes both behave the same. In the future only one format will be allowed.

Also included: clearer errors when the chemistry engine fails, per-zone surface equilibration, an error message when the tracer is named the same as a PHREEQC component (which previously failed with a confusing error), and tutorials that now render on every docs version (notebooks are committed instead of pulled from a fragile CI artifact).

Tested with new unit tests and the existing benchmarks; the model that hit this bug now completes a full run.