feat!: move SnapPlatformWatcher in SnapAccountService + add :ensureReady and use it in MultichainAccountService

[ccharly]

Explanation

Now that we have a central place for Snap accounts logic, we can move the SnapPlatformWatcher so we can use it (with an action) on the MultichainAccountService.

Also adding a new :ensureReady(snapId) action that will be improved in the future to make sure everything is ready when interacting with an account management Snap (migration, platform is ready, keyring is ready, etc...)

References

N/A

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
Introduces breaking messenger/API changes around Snap readiness (new required SnapAccountService:ensureReady, removed MultichainAccountService:ensureCanUseSnapPlatform), which can break consumers and affect Snap-based account operations if integration is incomplete.

Overview
Moves Snap platform readiness checks out of @metamask/multichain-account-service and into @metamask/snap-account-service. SnapAccountService now owns SnapPlatformWatcher, adds a new SnapAccountService:ensureReady(snapId) messenger action, and exposes config for onboarding gating and Snap-keyring wait timeout.

Updates multichain Snap providers to use the new readiness flow. MultichainAccountService drops its ensureCanUseSnapPlatform method/action and related config/types, and SnapAccountProvider.ensureCanUseSnapPlatform() is renamed to ensureReady() and now calls SnapAccountService:ensureReady. Tests, messenger policies, dependency graph, and package deps/tsconfig references are updated accordingly.

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