docs: document fiber scheduler concurrency

[jbeckwith-oai]

Summary

• document that the default Net::HTTP transport cooperates with an active Ruby fiber scheduler
• add an async usage example without introducing a runtime dependency
• add a localhost integration test proving two requests can make progress concurrently on one scheduler thread

Why

The SDK Surface Atlas classifies Ruby async support as a gap because the official SDK does not document a native path. The existing transport already uses scheduler-aware Ruby I/O, so this makes that behavior an explicit, tested SDK contract instead of adding a duplicate async client hierarchy.

Without an active scheduler, requests retain their existing blocking behavior.

Validation

• mise exec ruby@4.0.6 -- ./scripts/test — 525 runs, 1,712 assertions
• mise exec ruby@4.0.6 -- bundle exec rake lint:rubocop — 2,597 files, no offenses
• focused scheduler integration test on Ruby 3.3.12
• mise exec ruby@4.0.6 -- bundle exec rake typecheck

[openai-sdks]

OkTest Summary

✅ 237/237 SDK tests passed in 7.719s for Ruby SDK PR #357.

Test results — 42 files

Test	Result	Time
tests/chat-completions-complex-body.test.ts	✅ Passed	131ms
tests/chat-completions-create.test.ts	✅ Passed	194ms
tests/chat-completions-stream.test.ts	✅ Passed	115ms
tests/files-content-binary.test.ts	✅ Passed	139ms
tests/files-create-multipart.test.ts	✅ Passed	120ms
tests/files-list-pagination.test.ts	✅ Passed	134ms
tests/initialize-config.test.ts	✅ Passed	166ms
tests/instance-isolation.test.ts	✅ Passed	245ms
tests/models-list.test.ts	✅ Passed	140ms
tests/responses-background-lifecycle.test.ts	✅ Passed	170ms
tests/responses-body-method-errors.test.ts	✅ Passed	482ms
tests/responses-cancel-timeout.test.ts	✅ Passed	284ms
tests/responses-cancel.test.ts	✅ Passed	202ms
tests/responses-compact-retries.test.ts	✅ Passed	192ms
tests/responses-compact.test.ts	✅ Passed	185ms
tests/responses-create-advanced-stream.test.ts	✅ Passed	211ms
tests/responses-create-advanced.test.ts	✅ Passed	163ms
tests/responses-create-disconnect.test.ts	✅ Passed	125ms
tests/responses-create-errors.test.ts	✅ Passed	296ms
tests/responses-create-malformed-api-responses.test.ts	✅ Passed	113ms
tests/responses-create-retries.test.ts	✅ Passed	230ms
tests/responses-create-stream-failures.test.ts	✅ Passed	107ms
tests/responses-create-stream-timeout.test.ts	✅ Passed	192ms
tests/responses-create-stream-wire.test.ts	✅ Passed	1.657s
tests/responses-create-stream.test.ts	✅ Passed	132ms
tests/responses-create-terminal-states.test.ts	✅ Passed	228ms
tests/responses-create-timeout.test.ts	✅ Passed	267ms
tests/responses-create.test.ts	✅ Passed	217ms
tests/responses-delete.test.ts	✅ Passed	206ms
tests/responses-input-items-errors.test.ts	✅ Passed	165ms
tests/responses-input-items-list.test.ts	✅ Passed	162ms
tests/responses-input-items-options.test.ts	✅ Passed	169ms
tests/responses-input-tokens-count-timeout.test.ts	✅ Passed	290ms
tests/responses-input-tokens-count.test.ts	✅ Passed	194ms
tests/responses-malformed-inputs.test.ts	✅ Passed	1.816s
tests/responses-not-found-errors.test.ts	✅ Passed	344ms
tests/responses-parse.test.ts	✅ Passed	145ms
tests/responses-retrieve-retries.test.ts	✅ Passed	243ms
tests/responses-retrieve.test.ts	✅ Passed	227ms
tests/responses-stored-method-errors.test.ts	✅ Passed	577ms
tests/retry-behavior.test.ts	✅ Passed	1.95s
tests/sdk-error-shape.test.ts	✅ Passed	377ms

View OkTest run #31432247381

_{SDK merge (70aa56e32711) · head (5b90f02f24fe) · base (1c71a1eccdee) · OkTest (91635c6a2723)}

[HAYDEN-OAI]

Overall looks good. The Async example and transport behavior line up; I left one low-priority wording clarification inline.