fix: balance is set to 0 when importing non-evm asset

[juanmigdr]

Explanation

Balance is set to 0 when importing non-evm asset because assetsWithoutBalance.delete(assetId); is run conditionally when it shouldnt

References

https://consensyssoftware.atlassian.net/browse/ASSETS-1425

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, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

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Note

Fixes balance merge in MultichainBalancesController so importing a non‑EVM asset doesn’t reset its positive balance to 0; updates changelog.

• Balances:
  • Correct balance merge in src/MultichainBalancesController/MultichainBalancesController.ts by iterating with Object.keys(accountBalances) and unconditionally removing processed assets from assetsWithoutBalance.
    • Prevents imported non‑EVM assets with existing positive balances from being overwitten with 0.
• Docs:
  • Update CHANGELOG.md under Unreleased → Fixed to note the non‑EVM import balance issue.

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