fix(MultichainBalancesController): initialize balance entry when snap update arrives before account is in state

[salimtb]

Previously, #handleOnAccountBalancesUpdated would silently drop balance updates if state.balances[accountId] had not yet been populated (i.e. the key was absent). This caused a race condition where a Snap could send balances before updateBalance had created the entry, leaving the account with no persisted balance.

The handler now initializes the entry with ??= before merging, consistent with every other code path in the controller that writes to state.balances.

Explanation

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

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Note

Low Risk
Small, localized state-update change with added test coverage; low chance of unintended side effects beyond persisting previously-dropped balance updates.

Overview
Fixes a race in MultichainBalancesController where balances published via AccountsController:accountBalancesUpdated could be ignored if state.balances[accountId] was not yet initialized.

The handler now always initializes state.balances[accountId] before merging updates, and adds a unit test covering the “event arrives before state entry exists” case; the assets-controllers changelog is updated accordingly.

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