Fix optional null request body properties

[qybaihe]

Changes

• Fixes Request body { x?: null; } mishandled (regression from 0.16.0) #2717
• Preserve optional null properties when Writable<T> filters readOnly markers for request bodies.
• Adds an openapi-fetch regression type test for { x?: null } request bodies.

How to Review

• The marker helpers now treat never from NonNullable<null | undefined> as no marker, so optional null properties are retained.
• Existing readOnly/writeOnly request and response behavior is covered by the focused test run.

Checklist

• Unit tests updated
• docs/ updated (if necessary; not necessary for this type-only bugfix)
• pnpm run update:examples run (only applicable for openapi-typescript; not applicable)

Verification:

• pnpm --filter openapi-typescript-helpers run lint
• pnpm --filter openapi-fetch run lint
• pnpm --filter openapi-fetch run test

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🔨 Latest commit	ca66e52

[changeset-bot]

🦋 Changeset detected

Latest commit: ca66e52

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
openapi-typescript-helpers	Patch

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