Fix BaseModel nested model coercion

[bjedrocha]

Summary

Fixes nested BaseModel coercion so generated setters store successfully coerced nested model values instead of preserving the original raw hashes/arrays.

Details

Previously, the generated BaseModel setter computed coerced nested model values but stored the original raw input while marking the field as successfully coerced. As a result, nested BaseModel and ArrayOf[BaseModel] fields initialized from hashes could remain plain Hash values.

This updates the setter success check to store the coerced value when coercion succeeds, while preserving existing behaviour for already-correct values and failed conversions.

Before / after

require "openai"

class Item < OpenAI::BaseModel
  required :name, String
end

class Container < OpenAI::BaseModel
  optional :item, Item
  optional :items, OpenAI::ArrayOf[Item]
end

container = Container.new(
  item: { name: "single" },
  items: [{ name: "array" }]
)

Before this fix, the setter computed the coerced Item values but stored the raw input:

container.item.class
# => Hash

container.items.first.class
# => Hash

container.item.name
# => NoMethodError: undefined method `name' for an instance of Hash

After this fix, successful coercion is stored:

container.item.class
# => Item

container.items.first.class
# => Item

container.item.name
# => "single"

container.items.first.name
# => "array"