Union types are not handled correctly

Describe the bug

The internal types of Tanstack Form, DeepValue and DeepKeysAndValues don't handle union types correctly and seem to collapse them.

type Example = { id: string } | { id: undefined };

type Test = DeepKeysAndValues<Example>
//   ^? { key: "id"; value: string; }

const testObj: { id: string } | { id: undefined } = Math.random()
    ? { id: undefined }
    : { id: '' }

type idField = DeepValue<typeof testObj, 'id'>
// Type error - TS2322: Type undefined is not assignable to type string
const a: idField = undefined
const b: idField = 'zzz'

My use case was using a union type with withForm, this also happens with withFieldGroup

import type { MergeExclusive, RequiredDeep } from 'type-fest'

export type EditType = ContactType & ContactPatchType // ContactType has readonly properties like `id`, ContactPatchType has the properties that are editable
export type CreateType = RequiredDeep<ContactDataType> // ContactDataType contains properties that are used for initial create, so no `id` for example
type EitherType = MergeExclusive<CreateType, EditType>

export const ContactFormFields = withForm({
    defaultValues: {} as EitherType,
...

EditType contains {id: string} while CreateType does not. You would expect a form to understand that id may be string | undefined

Instead it becomes just {id: string}

Due to the incorrect type, this does not produce a type error but can fail in runtime since field.state.value may be undefined

<group.AppField name="id">
  {(field) => <div>ID: {field.state.value.length}</div>}
</group.AppField>

Your minimal, reproducible example

N/A

Steps to reproduce

^ See Above

Expected behavior

Union types to be handled properly

How often does this bug happen?

Every time

Screenshots or Videos

No response

Platform

"@tanstack/react-form": "1.23.7",

TanStack Form adapter

react-form

TanStack Form version

1.23.7

TypeScript version

5.9.3

Additional context

No response

Locate the internal DeepValue and DeepKeysAndValues definitions, then reproduce the issue with the union examples in the report and trace how their keys and values are inferred. Done means the resulting value type preserves undefined for the union and the withForm and withFieldGroup use cases no longer accept an incorrectly narrowed type.