feat(schema): preserve property ordering in generated JSON schema

[mikehostetler]

Summary

• preserve declared field order by emitting propertyOrdering for generated object schemas
• apply the ordering annotation to nested NimbleOptions map schemas and Zoi object schemas
• extend structured-output tests to verify the annotation survives request encoding

Testing

• mix test test/req_llm/schema_test.exs test/req_llm/schema_zoi_test.exs test/provider/openai/responses_api_unit_test.exs
• mix test test/req_llm/schema_zoi_metadata_test.exs test/provider/openai_structured_output_test.exs
• mix quality

Closes #596