Fix: assets controller startup empty accounts

[salimtb]

Explanation

Current state: On returning users (those with persisted accounts), AssetsController.#start() was called at keyring unlock time before AccountTreeController.init() had completed building the account tree. At that point, #selectedAccounts returned an empty array, so no subscriptions were set up and no asset fetch was triggered. The controller would then sit idle because:

• AccountTreeController:selectedAccountGroupChange only fires when the selected group changes — on returning users the persisted group is unchanged, so this event is skipped.
• AccountTreeController:accountTreeChange is not published by init() — it only fires when accounts are added/removed after initialization.

This meant asset balances, metadata, and prices were never loaded until the user manually switched accounts or networks.

Solution: Two changes working together:

1. Subscribe to AccountTreeController:stateChange — the base-controller event that is guaranteed to fire when init() calls this.update() to build the tree. When this fires, #updateActive() is called, which calls #start() now that accounts are available.

2. Make #start() idempotent — it now returns early if accounts/chains are not yet available (so the initial unlock call is a safe no-op), and also returns early if subscriptions are already active (so multiple subsequent stateChange firings — e.g. from snap accounts being added — don't trigger redundant fetches).

This follows the same pattern used by DeFiPositionsController, which also gets accounts lazily from AccountTreeController rather than eagerly at unlock time.

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

Medium Risk
Touches AssetsController startup/lifecycle logic and subscription gating, which could change when/ how often asset fetches run if the new idempotency checks miss an edge case.

Overview
Fixes a startup race where AssetsController could remain idle for returning users by additionally reacting to AccountTreeController:stateChange and re-evaluating active tracking once the account tree is initialized.

Makes #start() idempotent by returning early when no accounts/chains are available and when subscriptions are already active, preventing duplicate subscriptions/fetches from repeated events; adds a regression test for the unlock-then-tree-init flow and documents the fix in the assets-controller changelog.

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[Prithpal-Sooriya]

Can we use the state change selector (3rd param of the subscribe func)

[Prithpal-Sooriya]

Not a blocker, we'll receive a new event by accounts team which should remove the need for this state change event

[salimtb]

yes it's just a temp solution for now