Site Health entry for the last intent processed

## Feature Description

Support will get tickets saying the Google tag never appeared after someone came from the Google Ads console, and the first two questions are always the same: did the user reach the confirmation screen at all, and did they confirm it or walk away. Site Kit's section of the WordPress Site Health "Info" tab is where a user can copy that kind of detail into a support thread.

This issue records the last intent the site processed — which intent it was, what happened to it, and when — and shows it there. The requests it records are the ones served by the REST routes added in #13467. One field answers most of what a ticket needs.

Nothing from the intent payload is recorded. Site Health output is pasted into public support threads, and the tag ID, the Google Ads account name and the one-time code do not belong in one.

Link to the design doc: https://docs.google.com/document/d/1-8aikH7EzELWZCFuava3izGfhUVPiOFE3Oww4czjTW0/edit?tab=t.0

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## Acceptance criteria

* Site Kit's section of the WordPress Site Health "Info" tab shows a field for the last intent the site processed, giving the intent ID, the outcome and the date.
* The field is written when an intent is completed and when fetching or completing an intent fails, and its outcome distinguishes a completed intent from a failed one and names which failure it was.
* The field's value contains no tag ID, no Google Ads account name and no intent code.
* Before any intent has been processed on the site, the field says that none has been.
* The stored value is replaced each time, so exactly one intent is ever reported.

## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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