[9.x] Support nested fields in title_field and title_format

[duncanmcclean]

This pull request fixes an issue where setting title_field to a nested field (e.g., values_alt_title) didn't work correctly. Listings would either show blank titles or relationship fields would fail to display titles, depending on whether the arrow (meta->name) or underscore (meta_name) syntax was used.

This was happening because all call sites that read the title value used direct attribute access ($model->getAttribute(...) or $model->{$titleField}), which can't resolve values from nested JSON columns. Nested fields need to be read using data_get($model, "prefix.key").

This PR fixes it by adding three helper methods to Resource:

• titleValue(Model $model) — reads the title value from a model, using data_get() for nested fields
• titleDbColumn() — returns the database column path (e.g., values->alt_title) for use in query ordering
• setTitleValue(Model $model, $value) — writes a title value back to the model, handling nested JSON columns

All call sites that previously accessed the title value directly now use these methods instead.

Fixes #703