feat: add standard schema support for structured outputs and tools

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Confirm this is a feature request for the Node library and not the underlying OpenAI API.

• This is a feature request for the Node library

Describe the feature or improvement you're requesting

Summary

Add first-class Standard Schema support to the structured-output and tool helpers, so any conforming validation library — Zod, Valibot,
ArkType — works through a single integration, instead of a separate xResponseFormat / xFunction per library.

Why Standard Schema

One interface covers both jobs the helpers need:

Capability	Spec member	Notes
Parse model output	~standard.validate(value)	Universal across conforming libraries; replaces the per-library parse callback.
Generate JSON Schema	~standard.jsonSchema	The emerging Standard JSON Schema spec; used when a library exposes it.

Proposed shape

New helpers in src/helpers/standard-schema.ts, leaving the existing Zod helpers untouched:

• standardResponseFormat
• standardTextFormat
• standardFunction
• standardResponsesFunction

JSON Schema resolution — hybrid, in priority order:

1. Caller-supplied schema, else
2. ~standard.jsonSchema if the library exposes it, else
3. a clear, actionable error.

The result is always passed through the existing toStrictJsonSchema transform, so output stays Structured-Outputs-compatible.

Key properties:

• ✅ No new per-library dependencies — @standard-schema/spec is types-only.
• ✅ Validation routes through ~standard.validate.
• ✅ Full type inference via StandardSchemaV1.InferOutput.

Additional context

Follows the discussion in #1021

In that thread:

• @RobertCraigie noted the SDK shouldn't vendor a JSON-Schema converter per library.
• @fabian-hiller proposed Standard Schema to "reduce implementation effort and prevent vendor lock-in," later adding "it would be great if OpenAI could integrate it in the SDK."

A single Standard Schema integration addresses the converter-bloat concern directly.

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I'm happy to put together a PR with the implementation and tests (Valibot + ArkType end-to-end, plus a Zod parity test). Opening this issue first to confirm direction and scope before writing the code.