feat: AUS Perps Watchlist

[gambinish]

Explanation

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

[gambinish]

@aganglada One note on storage approach: why NotificationPreferences and not AssetsWatchlistBlob or some other purpose built storage blob like PerpsWatchlistBlob.

The AUS package actually has two separate storage endpoints:

1. getAssetsWatchlist / setAssetsWatchlist → /assets-watchlist — a dedicated, atomic watchlist blob ({ version: 1, assets: string[] })
2. getNotificationPreferences / putNotificationPreferences → /preferences/notifications — a combined blob covering wallet activity, marketing, perps, and social AI settings

At first glance assets-watchlist would be the cleaner fit — or potentially even a new purpose built storage endpoint perps-watchlist, but per the ticket spec we are following the existing schema. Using a separate endpoint would arguably be cleaner because we don't need to do this read-merge-write pattern, which runs the risk of accidentally clobbering our existing notification settings.

The tradeoff to be aware of with this approach: every watchlist toggle now does a GET → merge → PUT on the entire notification preferences blob. This is the same pattern NotificationServicesController uses for wallet account updates, so it's consistent with the existing codebase — but it does mean a concurrent notification preference change and a watchlist toggle could race. That's worth keeping in mind as the perps notification work continues.

Should we consider storing this under a dedicated endpoint in @metamask/authenticated-user-storage instead?

[gambinish]

If we do want to have this purpose built endpoint (I think we should), the changes needed would roughly follow the pattern of asset-watchlist:

1. New server endpoint in user storage API (likely a GET/PUT)
2. New methods and types on @metamask/authenticated-user-storage
3. Simplify this sync code in perps-controller, which would make things much cleaner