[snap-account-service] Remove extra account-tree-controller dep

[mcmire]

Explanation

In a future commit, we want to automatically synchronize the references field in each package's tsconfig files with its dependencies. Currently, we would have to make an exception for snap-account-service, as it lists account-tree-controller as a dependency but purposefully excludes it from its tsconfig files to avoid a circular reference. However, this manual bookkeeping is unnecessary, as snap-account-service already works around the circular dependency by copying types from account-tree-controller instead of importing them. So the dependency is unnecessary and we can drop it from snap-account-service's package.json.

References

Unblocks #8384.

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

Low Risk
Dependency and documentation-only change; no runtime or API code changes, with types already duplicated locally.

Overview
Removes @metamask/account-tree-controller from @metamask/snap-account-service’s declared dependencies and updates the monorepo dependency graph (README.md, yarn.lock). The package already avoids importing that controller and uses locally mirrored types in src/types.ts to break the account-tree-controller ↔ multichain-account-service ↔ snap-account-service cycle.

The long tsconfig comments that documented why account-tree-controller was omitted from project references are deleted now that the npm dependency no longer exists—references stay limited to keyring-controller and messenger. This aligns package.json with actual build/runtime usage so future automation can sync tsconfig references from dependencies without a special-case exception.

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