[Python] Define whether user-overridden to_pylist on Array subclasses is honored for nested children

[viirya]

In the review of #50327 it was noted (#50327 (comment)) that the bulk conversion does not consult a user-defined to_pylist override on ExtensionArray subclasses for nested children.

This was already inconsistent before #50327:

• ListScalar.as_py called child_values.to_pylist(), so an override on a list child's class was honored;
• StructScalar.as_py converted fields via per-element Scalars, so an override on a struct child's class was ignored.

Top-level overrides dispatch normally through Python method resolution in both the old and the new code, and ExtensionScalar.as_py / __arrow_ext_scalar_class__ — the documented customization point — is honored on all paths.

This issue is to decide the contract:

1. document that nested to_pylist overrides are not consulted (only ExtensionScalar.as_py is), or
2. detect custom to_pylist overrides and route nested conversion through them (sketch in the linked discussion).