prevent crash from missing account

[bergarces]

Explanation

Stop crash that occurs when there is a missing entry in the internalAccount object.

https://metamask.sentry.io/issues/7394639158/?project=273505&query=is%3Aunresolved%20Cannot%20read%20properties%20of%20undefined&referrer=issue-stream
https://metamask.sentry.io/issues/6977774458/?project=273505&query=is%3Aunresolved%20Cannot%20read%20properties%20of%20undefined&referrer=issue-stream

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 defensive change in a selector plus a unit test; behavior only changes for inconsistent state where an account ID is missing.

Overview
Prevents selectAssetsBySelectedAccountGroup from crashing when an accountTree group references an account ID that is missing from internalAccounts by skipping those entries during account mapping.

Adds a regression test covering the missing-account scenario and documents the fix in the assets-controllers changelog.

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