fix(form-core): exclude undefined from FormState errors type

[isaackaara]

The runtime code in FormApi.ts explicitly filters out undefined values from the error map when building the errors array:

fieldErrors = Object.values(currBaseMeta.errorMap ?? {}).filter(
  (val) => val !== undefined,
) as never

But the type definition allows undefined in the union, forcing consumers to add unnecessary undefined checks:

// Before: TypeScript thinks errors can contain undefined
errors.forEach(error => {
  console.log(error.length)  // TS error: Object is possibly 'undefined'
})

This wraps the error union types with NonNullable<> in both the FormState.errors type and the getAllErrors() return type, aligning the types with the runtime behavior.

// After: undefined is excluded from the type, matching runtime
errors.forEach(error => {
  console.log(error.length)  // Works
})

Fixes #2022

[changeset-bot]

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