fix: preserve binary multipart stream reads

[jbeckwith-oai]

Summary

• preserve arbitrary binary bytes when multipart request bodies yield a mix of UTF-8 and binary chunks
• honor IO#read(max_len, out_string) byte limits, buffering, EOF, negative-length, and output-buffer semantics for Enumerator-backed bodies
• retain native IO and StringIO delegation, including implicit to_int length conversion
• update RBI/RBS contracts and add focused regression coverage

This fixes #317 and consolidates the complete behavior needed from #275, #294, and #324 on the current SDK-owned ReadIOAdapter implementation.

Ownership

This is handwritten SDK runtime behavior introduced as an SDK-owned component in #345. It does not change OpenAPI inputs, Castiron facts, renderer rules, templates, or generator configuration.

Validation

• mise exec ruby@4.0.6 -- ./scripts/test — 531 runs, 1,736 assertions, 0 failures
• mise exec ruby@4.0.6 -- bundle exec rake lint — RuboCop, Sorbet, and 1,211 RBS files clean
• mise exec ruby@4.0.6 -- bundle exec rake build:gem
• installed-gem adapter probe
• Ruby 3.3.12 mixed-encoding compatibility probe
• 1,000 randomized read sequences compared against StringIO
• real local HTTP multipart upload with a 1 MiB binary file and UTF-8 prompt, verified byte-for-byte by SHA-256

[openai-sdks]

OkTest Summary

✅ 237/237 SDK tests passed in 8.777s for Ruby SDK PR #354.

Test results — 42 files

Test	Result	Time
tests/chat-completions-complex-body.test.ts	✅ Passed	175ms
tests/chat-completions-create.test.ts	✅ Passed	246ms
tests/chat-completions-stream.test.ts	✅ Passed	125ms
tests/files-content-binary.test.ts	✅ Passed	297ms
tests/files-create-multipart.test.ts	✅ Passed	261ms
tests/files-list-pagination.test.ts	✅ Passed	201ms
tests/initialize-config.test.ts	✅ Passed	199ms
tests/instance-isolation.test.ts	✅ Passed	260ms
tests/models-list.test.ts	✅ Passed	171ms
tests/responses-background-lifecycle.test.ts	✅ Passed	192ms
tests/responses-body-method-errors.test.ts	✅ Passed	400ms
tests/responses-cancel-timeout.test.ts	✅ Passed	294ms
tests/responses-cancel.test.ts	✅ Passed	203ms
tests/responses-compact-retries.test.ts	✅ Passed	243ms
tests/responses-compact.test.ts	✅ Passed	210ms
tests/responses-create-advanced-stream.test.ts	✅ Passed	129ms
tests/responses-create-advanced.test.ts	✅ Passed	243ms
tests/responses-create-disconnect.test.ts	✅ Passed	161ms
tests/responses-create-errors.test.ts	✅ Passed	259ms
tests/responses-create-malformed-api-responses.test.ts	✅ Passed	134ms
tests/responses-create-retries.test.ts	✅ Passed	268ms
tests/responses-create-stream-failures.test.ts	✅ Passed	134ms
tests/responses-create-stream-timeout.test.ts	✅ Passed	215ms
tests/responses-create-stream-wire.test.ts	✅ Passed	2.004s
tests/responses-create-stream.test.ts	✅ Passed	132ms
tests/responses-create-terminal-states.test.ts	✅ Passed	250ms
tests/responses-create-timeout.test.ts	✅ Passed	298ms
tests/responses-create.test.ts	✅ Passed	351ms
tests/responses-delete.test.ts	✅ Passed	232ms
tests/responses-input-items-errors.test.ts	✅ Passed	194ms
tests/responses-input-items-list.test.ts	✅ Passed	186ms
tests/responses-input-items-options.test.ts	✅ Passed	217ms
tests/responses-input-tokens-count-timeout.test.ts	✅ Passed	288ms
tests/responses-input-tokens-count.test.ts	✅ Passed	251ms
tests/responses-malformed-inputs.test.ts	✅ Passed	2.173s
tests/responses-not-found-errors.test.ts	✅ Passed	287ms
tests/responses-parse.test.ts	✅ Passed	210ms
tests/responses-retrieve-retries.test.ts	✅ Passed	280ms
tests/responses-retrieve.test.ts	✅ Passed	227ms
tests/responses-stored-method-errors.test.ts	✅ Passed	676ms
tests/retry-behavior.test.ts	✅ Passed	1.993s
tests/sdk-error-shape.test.ts	✅ Passed	351ms

View OkTest run #31431008263

_{SDK merge (e9a10f6ed8a3) · head (f711f4784e9b) · base (1c71a1eccdee) · OkTest (91635c6a2723)}