feat(ai-controllers): remove AiDigestController

[zone-live]

Explanation

AiDigestController was leftover Engine glue after digest freshness moved to React Query: empty state, empty-id checks, and a passthrough to AiDigestService.

This PR deletes the controller and related messenger/state exports. AiDigestService remains the fetch+validate layer (GET, 404→null, Superstruct). Empty assetIdentifier / front-page id now throw on the service instead of the controller. Clients construct AiDigestService themselves and own cache/freshness.

References

• TSA-1059
• Follows #10020 (controller became a cache-free passthrough)
• Mobile consumer PR will follow so you can point @metamask/ai-controllers at the preview build from this PR

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

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Note

Medium Risk
Breaking public API removal requires consumer migrations off messenger/controller wiring; service behavior change for empty IDs is minor but callers that relied on the controller layer must update.

Overview
BREAKING: Removes AiDigestController and all messenger/state/controller exports from @metamask/ai-controllers. Digest HTTP fetch and Superstruct validation stay on AiDigestService; callers instantiate the service directly and own caching/freshness (e.g. React Query).

Empty-input validation moves from the deleted controller into AiDigestService: searchDigest('') and fetchFrontPageItem('') now throw using the existing AiDigestControllerErrorMessage constants without hitting the API. Controller tests and messenger codegen scripts are removed; the package drops @metamask/base-controller and @metamask/messenger dependencies and README diagram edges for those links.

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