[Prithpal-Sooriya]

Description

Adds AssetsController.stale-balance-scenarios.integration.test.ts, an AssetsController-level integration suite covering all 12 stale-balance scenarios from the accounts API v6 integration decision doc (full and merge tables).

The suite boots the real AssetsController (with assetsAccountsApiV6: true) and mocks only the external boundaries, following the pattern of AssetsController.bsc-spam-token-filtering.integration.test.ts:

• HTTP APIs (Accounts API v6 + v2-supportedNetworks, tokens /v3/assets, prices /v3/spot-prices) are nocked from real captured responses in src/__fixtures__/stale-balance/api-responses/. Every fixture mutation is documented inline (balance doubling to model wallet movement between passes, occurrences stamping, chain omissions for RPC fallback).
• RPC / snap / lifecycle are exercised through mock messenger action handlers (NetworkController, SnapController, KeyringController, staking reads) and live events (AccountActivityService:balanceUpdated, AccountsController:accountBalancesUpdated, TransactionController:transactionConfirmed, KeyringController:unlock).

Scenario coverage (7 full + 5 merge): snap failure carry-over, skipped-asset last-amount copy, zero-seeding for never-seen visible assets, RPC fallback carry-over on failing balanceOf reads, disappearing zero-balance seeds, failed-fetch carry-over, fresh amounts on healthy fallback, event overlays, malformed-row handling, wrong-amount correction at the next full poll, snap-event overlay + refresh, and staked ETH on mainnet + Hoodi across an unlock lifecycle with a staking deposit.

Test plan

• yarn workspace @metamask/assets-controller run test
  • New suite: 12/12 scenarios pass
  • Full package suite: 39 suites / 1172 tests pass
• yarn lint checks on the new files: oxlint and oxfmt clean
• yarn build (root, full monorepo type-check) passes

Notes for reviewers

• Fixture balance-row shape includes unit, required by the keyring API's Balance struct via strictMask (@metamask/keyring-api); the controller discards it downstream.
• Native ETH on Hoodi is keyed under the zero-address ERC-20 encoding (eip155:560048/erc20:0x0000000000000000000000000000000000000000) — the controller's documented fallback for EVM chains absent from its native-asset seed map, which excludes testnets.
• The Multicall3 aggregate3 fixture encodes the contract's real return shape — Result[] only, with no leading blockNumber (only aggregate/tryAggregate return one). An earlier draft of the fixture encoded (uint256, Result[]), which made every batch decode throw and silently fall through to single balanceOf calls; caught in review and fixed, so the scenarios exercise the batched read path. MulticallClient's decoder is correct as shipped — no product bug here.

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Note

Low Risk
Test and fixture additions only; no runtime balance or merge logic changes in production code.

Overview
Adds end-to-end integration coverage for all 12 stale-balance scenarios from the accounts API v6 decision doc (full and merge tables), without changing AssetsController production code.

The new suite boots the real controller with assetsAccountsApiV6: true and replaces external boundaries only: nock interceptors serve captured Accounts/Tokens/Prices API responses (with scenario knobs like unprocessedNetworks, balance overrides, and stamped token occurrences so spam filtering does not dominate), while messenger mocks supply BNB/mainnet/Hoodi RPC (Multicall3 + staking reads), the Solana keyring snap, wallet lifecycle, and live events (AccountActivityService:balanceUpdated, snap balance updates, staking transactionConfirmed).

Twelve tests assert last-known vs fresh behavior when the snap fails or omits assets, BNB Chain falls back to RPC (including partial failures and zero-token drops), activity events merge or are ignored when malformed, and staked ETH on mainnet/Hoodi survives token polls after a vault deposit.

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[Prithpal-Sooriya]

Note on the failing "Validate changelog diffs" check: this failure is a known CI limitation for fork PRs, not an issue with this PR's changes.

• The job log shows it failed at ref resolution, before any changelog validation ran: fatal: Not a valid object name origin/feature/use-memory-see-this-sp0 (exit 128).
• This PR is opened from a fork, so its head branch does not exist under origin/* in the base-repo clone that the job checks out.
• All other lint/build/test jobs pass on the head commit, including lint:oxlint, lint:misc:check, and the per-package Validate changelog jobs. No CHANGELOG.md files are modified in this diff (the PR carries the no-changelog label), so once the upstream fix for the action lands, a CI re-run will pass this job.

Upstream fix in flight: dbced1f5f2e — "ci: resolve changelog checks against the pull request ref" (branch fix/changelog-checks-fork-prs), which resolves the PR head via refs/pull/<number>/head instead of origin/<branch>.