feat: aus leaderboard preferences

[Bigshmow]

Explanation

Adds getLeaderboardPreferences and setLeaderboardPreferences to AuthenticatedUserStorageService, backed by a new dedicated AUS namespace GET/PUT /api/v1/preferences/leaderboard with the blob shape { version: 1, optedOut: boolean}. This is the storage layer for the Top Traders leaderboard opt-out feature: the client persists the user's opt-out choice so the leaderboard UI can preselect the post-opt-out state on launch and across devices.

The preference is stored as its own blob rather than as a field on the existing NotificationPreferences blob. The notification-preferences blob only exists once a user has enabled notifications (it's seeded on notification setup and a full-blob write), so nesting the flag there leaves users who never touched notifications with nowhere to persist it. A dedicated blob is created-on-first-write, so opt-out persists for everyone with no coupling to notifications. The implementation mirrors the existing assets-watchlist feature exactly (validator + assert, get/set methods, messenger actions, exports, tests).

Nothing else in the package changes; this is purely additive.

References

• Related to TSA-834 — dedicated leaderboard-preferences AUS blob.
• Consumed by the MetaMask mobile client via TSA-830 (leaderboard opt-out client flow).
• Depends on the corresponding backend endpoint in va-mmcx-notify-users-storage (/preferences/leaderboard), which is not yet deployed — unit tests mock the HTTP layer, so this SDK can't be exercised end-to-end until that ships. Draft until then.

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