fix(zod): convert oneOf unions to anyOf

[FU-max-boop]

Summary

Fixes #1709.

Zod v4 emits oneOf for discriminated unions, but OpenAI strict JSON schemas do not allow oneOf. The Zod v4 helper path already runs schemas through toStrictJsonSchema, so this converts oneOf unions into strict-compatible anyOf unions during that normalization pass.

The regression test covers a Zod discriminated union through zodResponseFormat and asserts the final schema no longer contains oneOf.

Tests

• npx --yes [email protected] test tests/helpers/zod.test.ts --runInBand
• npx --yes [email protected] --check src/lib/transform.ts tests/helpers/zod.test.ts
• ./node_modules/.bin/eslint src/lib/transform.ts tests/helpers/zod.test.ts
• git diff --check

Note: I also ran ./node_modules/typescript/bin/tsc --noEmit; it surfaced missing optional example dependencies in this checkout (@azure/identity, express, next). The only PR-local type issue it reported was fixed before the checks above were rerun.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve existing anyOf constraints when converting oneOf

When a schema node already has an anyOf, JSON Schema applies the existing anyOf and oneOf as separate constraints, but this merge turns all branches into alternatives. For callers of toStrictJsonSchema that pass a valid schema containing both keywords on the same node, values that satisfy only a former oneOf branch can now be accepted even if they fail the original anyOf constraint, broadening the response schema instead of preserving it.

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[FU-max-boop]

Fixed in 8882406f: when a schema node already has anyOf, the converted oneOf is kept as a separate allOf constraint instead of merging branches into the existing anyOf. I also added a regression test covering the mixed anyOf + oneOf case.