Support dotted embedding paths in nested sourced_from relationship chains

[ellisandrews-toast]

Why

The nested sourced_from feature is meant to work for any schema, but as merged it only handled embedding fields that sit directly on each relationship's parent type. Real schemas can nest the embedded type one or more object levels down — e.g. a list of Players reached via roster.players rather than a top-level players field. In that case, resolving the parent_relationship failed with a "field does not exist" error, so you simply couldn't source into a nested element unless it happened to be a direct field.

This closes that gap: an embedding field referenced via parent_field_name may now be a dotted path that descends through intermediate object fields. The embedding path is expanded into one navigation segment per component, so the sourced data can reach elements nested arbitrarily deep.

Scope / behavior notes

• Intermediate lists are rejected (only the final embedding field may be a list), with an error that points at the supported alternative — model the intermediate list as its own parent_relationship link, so each list level is matched by its own foreign key. Multi-link chains already support arbitrary list nesting this way.
• Naming convention: parent_field_name now resolves by the field's declared name, consistent with how sourced_from source paths and equivalent_field already resolve. Previously it incidentally required the internal name_in_index. Note "declared name" is not the same as "exposed in GraphQL" — indexing_only embedding fields are still resolvable, since they have a declared name even though they're hidden from the schema.

Testing

Unit coverage added for: object→list and object-leaf dotted paths, public-name resolution with a distinct name_in_index, the intermediate-list rejection, an unresolvable path, and a deeply nested chain that mixes object hops, multiple list links, and name_in_index renames.

[myronmarston]

LGTM! I pushed a commit with a recommended refactor. I also left some inline suggestions below which I'll apply shortly so we can get this merged. For the parent_field_name renaming I'll tackle that in a follow up PR.