Re-validate the root model only once per _update_fields call.

[copybara-service]

Re-validate the root model only once per _update_fields call.

BaseModelFrozen._update_fields walked from every mutated submodel up to the
root and called m.__class__.from_dict(m.to_dict()) on each node along the way.
Because from_dict(to_dict()) already re-validates the whole subtree
recursively, and because the ancestral paths of sibling submodels overlap, this
re-validated the same models many times over. Updating the 18 parameters that a
pulse optimization step writes triggered roughly 54 full serialization and
validation passes, 18 of which were over the entire ToraxConfig tree.

Deduplicate the traversal: clear the functools.cached_property caches once per
unique ancestral node, then re-validate the root model exactly once. Validation
coverage is unchanged, since validating the root recursively validates every
mutated and ancestral submodel.

This reduces the time to apply a 18-parameter update to a ToraxConfig from
3.88s to 0.24s (16x) in an optimized build, and from 12.18s to 0.75s in a
fastbuild. For a TORAX simulation driven by an external optimizer this is pure
startup overhead paid on every trial.