fix: coerce nested BaseModel fields

[bjedrocha]

Summary

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

Root cause

The generated setter assigned the return value of Hash#store to error. Because Hash#store returns the value it stores, the successful branch that stored coerced was unreachable.

Composite converters also shared one mutable error slot. A later successful field or collection item could clear an earlier error, while a rejected union variant could overwrite the error state of the selected variant.

Changes

• Store successfully coerced nested models and collections.
• Preserve identity for values that already satisfy their target type.
• Preserve raw input and conversion errors when any model field, array item, or map value fails coercion.
• Isolate errors from rejected union variants and retain the selected variant's state.
• Preserve existing model identity during direct BaseModel coercion.
• Preserve union strictness for discriminator-resolved variants.

Before / after

require "openai"

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

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

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

Before this fix, the generated setter returned raw hashes:

container.item.class
# => Hash

container.items.first.class
# => Hash

After this fix, successful coercion is stored:

container.item.class
# => Item

container.items.first.class
# => Item

Validation

• ./scripts/test — 437 runs, 1,359 assertions, 0 failures
• ./scripts/lint — RuboCop, Sorbet, and Steep all pass

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[HAYDEN-OAI]

Reviewed the nested model coercion changes, including collection error aggregation, union isolation, identity preservation, and the follow-up nullable collection fix. No substantive issues found.