fix: encode multipart array and nested fields

[jbeckwith-oai]

Summary

• encode multipart arrays recursively as field[], including both primitive and file arrays
• flatten nested multipart objects with bracket notation, including arrays of nested objects
• preserve scalar field names and streaming file behavior
• escape both name and filename Content-Disposition parameters
• keep the fix entirely in the openai-ruby runtime; no generated model, OpenAPI, or Castiron changes are needed

Root cause

The shared multipart writer repeated top-level primitive and file arrays using the scalar field name and serialized nested hashes as JSON parts. The API's form parser expects the same global brackets format used by openai-python and by the transformed OpenAPI examples.

Before:

• timestamp_granularities=word and timestamp_granularities=segment
• duplicate scalar image file parts
• one JSON expires_after part

After:

• timestamp_granularities[]=word and timestamp_granularities[]=segment
• duplicate image[] file parts, while a single image remains image
• expires_after[anchor]=created_at and expires_after[seconds]=3600

The implementation replaces the old top-level array special case with one recursive field writer. It preserves insertion order, array order, duplicate values, and streaming file contents without materializing a flattened body.

This consolidates and supersedes #249, #311, and #339.

Fixes #201.
Fixes #212.
Fixes #253.

Validation

• mise exec ruby@4.0.6 -- bundle exec ruby -Itest test/openai/internal/util_test.rb — 45 runs, 178 assertions
• mise exec ruby@4.0.6 -- ./scripts/test — 522 runs, 1,709 assertions
• mise exec ruby@4.0.6 -- bundle exec rake lint — 2,596 files, no RuboCop offenses; Sorbet clean; 1,211 RBS files valid
• mise exec ruby@4.0.6 -- bundle exec rake build:gem — built openai-0.78.0.gem
• RBI and RBS formatting plus git diff --check
• thermo-nuclear maintainability review and a separate correctness/readability/architecture/security/performance review, repeated after findings until clean

[openai-sdks]

OkTest Summary

✅ 237/237 SDK tests passed in 8.488s for Ruby SDK PR #348.

Test results — 42 files

Test	Result	Time
tests/chat-completions-complex-body.test.ts	✅ Passed	163ms
tests/chat-completions-create.test.ts	✅ Passed	236ms
tests/chat-completions-stream.test.ts	✅ Passed	129ms
tests/files-content-binary.test.ts	✅ Passed	225ms
tests/files-create-multipart.test.ts	✅ Passed	242ms
tests/files-list-pagination.test.ts	✅ Passed	171ms
tests/initialize-config.test.ts	✅ Passed	153ms
tests/instance-isolation.test.ts	✅ Passed	239ms
tests/models-list.test.ts	✅ Passed	173ms
tests/responses-background-lifecycle.test.ts	✅ Passed	207ms
tests/responses-body-method-errors.test.ts	✅ Passed	448ms
tests/responses-cancel-timeout.test.ts	✅ Passed	207ms
tests/responses-cancel.test.ts	✅ Passed	231ms
tests/responses-compact-retries.test.ts	✅ Passed	228ms
tests/responses-compact.test.ts	✅ Passed	227ms
tests/responses-create-advanced-stream.test.ts	✅ Passed	135ms
tests/responses-create-advanced.test.ts	✅ Passed	205ms
tests/responses-create-disconnect.test.ts	✅ Passed	160ms
tests/responses-create-errors.test.ts	✅ Passed	264ms
tests/responses-create-malformed-api-responses.test.ts	✅ Passed	130ms
tests/responses-create-retries.test.ts	✅ Passed	275ms
tests/responses-create-stream-failures.test.ts	✅ Passed	139ms
tests/responses-create-stream-timeout.test.ts	✅ Passed	218ms
tests/responses-create-stream-wire.test.ts	✅ Passed	1.973s
tests/responses-create-stream.test.ts	✅ Passed	84ms
tests/responses-create-terminal-states.test.ts	✅ Passed	304ms
tests/responses-create-timeout.test.ts	✅ Passed	218ms
tests/responses-create.test.ts	✅ Passed	290ms
tests/responses-delete.test.ts	✅ Passed	234ms
tests/responses-input-items-errors.test.ts	✅ Passed	188ms
tests/responses-input-items-list.test.ts	✅ Passed	188ms
tests/responses-input-items-options.test.ts	✅ Passed	251ms
tests/responses-input-tokens-count-timeout.test.ts	✅ Passed	290ms
tests/responses-input-tokens-count.test.ts	✅ Passed	261ms
tests/responses-malformed-inputs.test.ts	✅ Passed	2.128s
tests/responses-not-found-errors.test.ts	✅ Passed	323ms
tests/responses-parse.test.ts	✅ Passed	182ms
tests/responses-retrieve-retries.test.ts	✅ Passed	257ms
tests/responses-retrieve.test.ts	✅ Passed	268ms
tests/responses-stored-method-errors.test.ts	✅ Passed	616ms
tests/retry-behavior.test.ts	✅ Passed	1.952s
tests/sdk-error-shape.test.ts	✅ Passed	331ms

View OkTest run #31335329567

_{SDK merge (85ffda68ff94) · head (9d04855c556e) · base (20fbb0942991) · OkTest (91635c6a2723)}