fix(checkbox): show labels after page navigation (#31062)

Issue number: resolves #31052

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## What is the current behavior?
After a page navigation, ion-checkbox's `onslotchange` event fires
before the element's `textContent` has been updated. It is called again
after `textContent` becomes readable on Safari, but is not called again
after the `textContent` becomes readable on Chrome and Firefox.

## What is the new behavior?
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this PR. -->

- Uses `MutationObserver` instead of `onslotchange` and fires
specifically on character data changes. This ensures `hasLabelContent`
is up to date.
- `MutationObserver` does not fire on load, so `hasLabelContent` is
initialized in `connectedCallback`

## Does this introduce a breaking change?

- [ ] Yes
- [X] No

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