test(assets-controller): repro stale balance for fully-spent tokens (…

[Kriys94]

…ASSETS-3856)

The Accounts API reports a zero balance by omission: a token the account no longer holds is absent from balance responses rather than returned as "0". Subscription poll responses are applied with updateMode 'merge' — an object spread that can only add or overwrite keys — so an omitted token keeps its last non-zero balance through every poll cycle until app restart.

Adds a repro driving the real subscription pipeline (unlock, startup fetch, AccountsApiDataSource poll) against a mocked Accounts API client, mirroring the reported scenario: a Linea account max-swaps USDC to USDT and the API omits USDC from subsequent polls.

NOTE: the first test asserts the DESIRED behavior and intentionally FAILS on current code (expected "0", received "100"); it becomes the regression test once a fix lands. The second test (explicit zero through the same pipeline) passes, isolating the defect to omission handling.

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