[saved objects] Improve type safety in the saved objects repository

[TinaHeiligers]

The is an existing pattern of using // @ts-expect-error MultiGetHit._source is optional comments in the saved object repository code that weakens the type safety.
We should use a type guard instead

/**
 * Type guard to check if an item is a GetGetResult.
 */
export function isGetGetResult<TDocument = unknown>(
  item: MgetResponseItem<TDocument>
): item is GetGetResult<TDocument> {
  return (item as MgetMultiGetError).error === undefined;
}

Originally posted by @rudolf in #224411 (comment)

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[Ankit-raj-11]

Hi @TinaHeiligers 👋,

I’ve gone through the issue and fully understand the problem.
The current use of // @ts-expect-error in the Saved Objects repository weakens type safety, and I agree that replacing it with a proper type guard is the right approach.

---

💡 My Approach

I plan to:

1. Identify all instances where @ts-expect-error is used (mainly around MultiGetHit._source).

2. Implement a type guard like:

   export function isGetGetResult<TDocument = unknown>(
     item: MgetResponseItem<TDocument>
   ): item is GetGetResult<TDocument> {
     return (item as MgetMultiGetError).error === undefined;
   }

3. Refactor the existing logic to use this type guard instead of @ts-expect-error.

4. Run tsc and related tests to ensure everything compiles and works correctly.

---

Would you please assign this issue to me?
I’d love to work on it and submit a PR with the fix.

[TinaHeiligers]

@Ankit-raj-11 thank you for stepping up to contribute!
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[Ankit-raj-11]

Hi @TinaHeiligers 👋,

I’ve submitted the PR #243130 for the type guard improvement.
Whenever you get a moment, could you please review it?

If you feel any changes or improvements are needed, I’ll be happy to update the PR.
Thanks a lot for your time and guidance! 🙏