fix(serializer): honour nested includes in lazy_load_data

[tvdeyen]

jsonapi-serializer's lazy_load_data is meant to omit a relationship's data linkage unless that relationship was requested via include. It works for top-level relationships but is broken for nested ones: a relationship emits linkage unless lazy_load_data && !included, where included = includes_list.include?(key) — but the top-level includes are a flat list of dotted symbols (primary_taxon.ancestors), and get_included_records hands each sideloaded record its parent's includes_list rather than the scoped remainder. So enabling lazy_load_data suppresses linkage for every relationship of a nested resource, including the ones that were requested — making it unusable beyond the top level.

This prepends a patch onto FastJsonapi::SerializationCore that:

• matches included against the base key of each requested include path (so primary_taxon.ancestors marks :primary_taxon, and the taxon's own :ancestors), and
• threads the correctly-scoped remainder into each sideloaded record.

With it, lazy_load_data: true emits linkage only for relationships actually requested at that resource's position in the include tree — which lets a serializer stop emitting the unrequested back-references (a taxon's children when only ancestors was asked for, an option value's option_type, …) that otherwise bloat responses and create cycles for consumers.

Scope / safety

• It does not enable lazy_load_data anywhere. It's a pure bug fix and is completely inert for relationships that don't set lazy_load_data: true (their linkage is emitted regardless).
• It patches a gem constant, so it's loaded at boot from the engine via require (flickwerk/Zeitwerk on_load only fires for app-autoloaded constants, not gem code).

Testing
Spec added under spec/serializers/… reproducing the nested case (requested relationship keeps data, unrequested nested relationship drops it, and non-lazy relationships are untouched). The fix was also validated directly against jsonapi-serializer 2.2.0 in isolation.

[mamhoff]

Amazing.