[mattt]

This PR fixes structured output schemas for OpenAILanguageModel and OpenResponsesLanguageModel, which send them with strict: true. OpenAI's strict mode requires every property of every object to be in required, with a null type for an optional one, but the schema conversion applied this only to the root object. An optional property of a nested object, of the objects in an array, or of an enum case such as case note(String?) was left out of required. Optional properties of the root were listed in required but did not allow null, so the model could not leave them empty.

Both providers now share one conversion that applies the rules to every object in the schema, through properties, array items, and anyOf and oneOf choices. An optional property becomes required and allows null: a primitive type without listed values gets "type": ["string", "null"], as in OpenAI's examples, and an enum string, object, array, or union becomes a union with {"type": "null"}. Decoding already reads null as nil. Other providers are unchanged.

Resolves #327