[9.x] Fix nested JSON fields in title_field and title_format

[duncanmcclean]

This pull request fixes an issue where the title_field resource option and the title_format fieldtype option couldn't reference fields nested inside JSON columns.

This was happening because every consumer of title_field read the value with $model->getAttribute($titleField) or magic property access, both of which return null for a nested handle like address_street_name. The one place it did work was the relationship fieldtype's selected items, which resolve the title through augmentation instead.

The issue also asked which syntax is correct, so for the record: it's the nested handle (address_street_name), same as anywhere else in a blueprint. The arrow form (address->street_name) is the database column Runway generates internally, and was never a valid blueprint handle — which is why the listing title rendered but didn't link.

This PR fixes it by adding getFieldValue() to the HasRunwayResource trait as the read-side counterpart to getFieldColumn(), then routing titles through it: listings, the edit form, publish form values, revisions, search results and relationship columns. Query-side paths now convert the handle to a column, so sorting in the stack selector, the relationship filter, runwaySearch and duplication work with a nested title field too.

This PR also fixes:

• Nested fields whose prefix contains an underscore (eg. external_links) always augmenting to null, since AugmentedModel derived the prefix with Str::before($handle, '_'). This affects title_format, which reads values through augmentation.
• Nested fields being excluded from runwaySearch entirely, since the "does this map to a real column?" check tested the field handle instead of the column it maps to.
• Nested fields listed in a search index's fields being indexed as null, since Searchable also read them with magic property access.

While here, getColumnForField() has been renamed to getFieldColumn() to match. The old name still works, but is deprecated.

The docs for both options have been updated to mention nested fields.

Fixes #703

[github-actions]

Released as part of v9.7.0.