Fix Zod v4 oneOf unions in strict schemas

[kiwigitops]

Closes #1709

Zod v4 can emit oneOf for discriminated unions, but strict OpenAI schemas support anyOf instead. This converts oneOf unions before the existing strict-schema recursion runs, so nested discriminated unions are still processed normally.

Verification:

• yarn jest tests/helpers/zod.test.ts --runInBand
• yarn jest tests/lib/transform.test.ts tests/helpers/zod.test.ts --runInBand
• git diff --check

[chatgpt-codex-connector]

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[kiwigitops]

Quick follow-up on this. The oneOf semantics issue was addressed in the latest commit; happy to adjust the schema transform or test coverage if this needs a different shape.

[HAYDEN-OAI]

@kiwigitops thanks for sticking with this and for getting the original fix moving. appreciate your patience while we worked through the schema semantics and brought the branch current. merging now! 💚