feat: adds ai digest controller

[Bigshmow]

Explanation

Adds AiDigestController to the @metamask/ai-controllers package for fetching and caching AI-generated asset digests.

What it does:

• AiDigestService - Fetches digest data from GET /digests/assets/:assetId/latest
• AiDigestController - Manages state, caching (10 min TTL, 50 entry max), and FIFO eviction
• Follows existing controller patterns (BaseController, messenger actions, etc.)

Design decisions:

• DigestData type mirrors the API response structure from va-mmcx-digest-api - we'll iterate inline as the API evolves
• baseUrl is required (no default) - consuming apps must provide their API endpoint
• Eviction handles stale entries, error/loading states, and invalid entries from corrupted persistence

This is a V1 implementation - the API response shape may evolve as the backend is finalized.

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

---

Note

Medium Risk
Adds new persisted controller state and a network-backed fetch flow; correctness of TTL/FIFO eviction and API error handling affects UI behavior and storage growth, but does not touch auth or funds.

Overview
Adds a new AiDigestController that fetches asset digests via an injected DigestService, persists them in controller state, and exposes messenger actions for fetchDigest, clearDigest, and clearAllDigests.

Implements caching with a 10-minute TTL (CACHE_DURATION_MS) and a 50-entry max (MAX_CACHE_ENTRIES) with stale + FIFO eviction, and introduces AiDigestService to call GET /digests/assets/:assetId/latest with explicit error handling.

Updates @metamask/ai-controllers exports, tests (replaces the placeholder greeter test), adds base-controller/messenger deps and TS project references, and notes the addition in the changelog.

^{Written by Cursor Bugbot for commit 6a24fd3. This will update automatically on new commits. Configure here.}

[Bigshmow]

re this PR's failing lint job: Is this dedupe issue a known issue? resolved

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.