fix(AccountTrackerController): prevent balance wipe during account sync

[Prithpal-Sooriya]

Explanation

Added a test to ensure that existing balances are preserved when syncing accounts, specifically when the selected chain has no state entry. Updated the controller logic to only initialize new addresses with a zero balance if they do not already exist in the accountsByChainId state, preventing unintended overwrites of existing balances.

References

MetaMask/metamask-extension#40474

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
Touches core balance state mutation logic; a small conditional change could affect how new accounts/chains are initialized, so regressions would surface as missing/incorrect balances across networks.

Overview
Prevents AccountTrackerController.#syncAccounts from overwriting existing address entries (and their stakedBalance) when syncing accounts for a chain that previously had no accountsByChainId entry, by only initializing { balance: '0x0' } when the address key is absent.

Adds a regression test covering the scenario where refreshing on a new chain should initialize the new chain’s account state without wiping balances on other chains, and documents the fix in the assets-controllers changelog.

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[salimtb]

nice catch