fix: preserve Date values in optimistic updates

[louzhedong]

Fixes #10478.

Summary

useUpdate and useUpdateMany previously used JSON.parse(JSON.stringify(...)) to remove undefined values from optimistic payloads. That also converted Date values to strings, which caused optimistic cache updates to lose date types.

This change replaces that serialization step with a recursive removeUndefined helper that:

• removes undefined values from plain objects
• preserves Date instances and other non-plain values
• keeps the existing optimistic update behavior intact

Tests

• yarn.cmd test-unit packages/ra-core/src/util/removeUndefined.spec.ts packages/ra-core/src/dataProvider/useUpdate.spec.tsx packages/ra-core/src/dataProvider/useUpdateMany.spec.tsx --runInBand --watch=false --testNamePattern "preserves Date values in the cache|does not erase values if the payload contains undefined values|removeUndefined"
• yarn.cmd workspace ra-core build

Notes

I added a regression test that verifies optimistic cache updates keep Date values as Date instances, even when the payload also contains undefined fields.

[louzhedong]

Quick follow-up: I re-checked the branch locally and the targeted tests are still green on my side. The only failing GitHub checks are the Vercel deployment authorizations, which look external to the code change. If you'd like, I can re-run or narrow anything further once there's a maintainer review comment.