…ureReady` and use it in `MultichainAccountService` (#8715)

## 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](https://github.com/MetaMask/core/tree/main/docs/processes/updating-changelogs.md)
- [ ] I've introduced [breaking
changes](https://github.com/MetaMask/core/tree/main/docs/processes/breaking-changes.md)
in this PR and have prepared draft pull requests for clients and
consumer packages to resolve them

<!-- CURSOR_SUMMARY -->
---

> [!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.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
c41e954. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->