feat: Add onSettled callback to commitMutation and useMutation

[jungwoo3490]

I added an onSettled callback to commitMutation and useMutation, similar to React Query's onSettled. It fires after both onCompleted and onError, removing the need to duplicate "always-run" logic across both callbacks (e.g. hiding a loading indicator, closing a modal, sending analytics).

Signature:

onSettled?: (
  response: TData | null,       // response data on success, null on error
  errors: PayloadError[] | null, // payload-level errors (can exist on success too)
  error: Error | null,           // network/execution error, null on success
) => void

Behavior:

Success: onCompleted(response, payloadErrors) -> onSettled(response, payloadErrors, null)
Error: onError(err) -> onSettled(null, null, err)

why invocation logic lives only in commitMutation

useMutation is a React wrapper around commitMutation. It intercepts onCompleted and onError solely to inject cleanup logic (in-flight state management). onSettled fires after that cleanup has already run, so there is nothing for useMutation to add.

Instead, useMutation passes onSettled through to commitMutation naturally via ...config spread, and commitMutation handles the invocation directly. This keeps the callback logic in one place and avoids duplicating it across both layers.

Test

• commitMutation: success, success with payload errors, network error, onSettled-only (no onCompleted/onError)
• useMutation: same 4 cases above