fix(responses): correct error message in finalResponse and safely accumulate string deltas

[hsusul]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

• Fix error message thrown by ResponseStream.finalResponse() when a stream ends prematurely without producing a response (stream ended without producing a Response instead of stream ended without producing a ChatCompletion).
• Fix typo in ResponseStream.finalResponse() docstring (REsponse -> Response).
• Safely handle nullish initial values when accumulating string deltas in ResponseAccumulator.ts across output_text, refusal, function_call_arguments, reasoning_text, reasoning_summary_text, custom_tool_call_input, and mcp_call_arguments events using (target ?? '') + event.delta.
• Add unit tests for finalResponse() error message and ResponseAccumulator string delta accumulation.

Additional context & links

Problem

1. ResponseStream belongs to the Responses API. When finalResponse() is called on a stream that ended prematurely without producing a Response, it previously threw OpenAIError('stream ended without producing a ChatCompletion').
2. When stream delta events (response.function_call_arguments.delta, response.refusal.delta, etc.) arrive on uninitialized string properties, += event.delta resulted in "undefined" prefixing the delta content.

Root cause

1. Copy-paste typo from ChatCompletionStream.ts in ResponseStream.ts.
2. Missing nullish fallback (str ?? '') on string property delta accumulation in ResponseAccumulator.ts.

Verification

• pnpm test tests/lib/ResponseStream.test.ts (Pass)
• pnpm build (Pass)
• pnpm lint (Pass)

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 5daeffe0fe

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Leave arguments uninitialized in the regression test

This initializes arguments to an empty string, so the test never exercises the nullish fallback it claims to cover: the previous output.arguments += event.delta implementation also produces '{"loc' and passes this assertion. Simulate the reported runtime payload with the property omitted or set to undefined (using an appropriate cast since the generated type requires a string) so the test fails if the fix is reverted.

Useful? React with 👍 / 👎.

[jbeckwith-oai]

Requesting one test correction before merge:

tests/lib/ResponseStream.test.ts:276 — the regression test never exercises the nullish case. The fixture initializes arguments to '', so the pre-change implementation (output.arguments += event.delta) already produces '{"loc' and passes this assertion. The test therefore cannot catch a regression in the new ?? '' behavior. Please model the reported wire payload by actually omitting/setting the field to undefined at a narrow cast boundary. Since this PR changes eight accumulator targets, table-driven coverage in the existing focused ResponseAccumulator.test.ts for the affected event families would make the contract explicit without duplicating full stream setup.

The runtime changes themselves are small, direct, and otherwise look safe.

Validation at exact head 5daeffe0febbdcb0d59672e8ee3eae3eab2fc79a: focused ResponseStream and ResponseAccumulator tests passed (14 total); changed-file ESLint and Prettier checks passed; git diff --check passed. Full repository type-check was blocked by missing optional example-only dependencies (@azure/identity, express, and next) in the available dependency tree. No hosted checks are attached to the PR.