fix(AssetsController): Clean code (#7867)

[Kriys94]

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

---

Note

Medium Risk
Touches core subscription/chain-assignment logic and introduces a
breaking API contract for data-source callbacks, which could cause missed subscriptions or runtime errors if any caller isn’t updated.

Overview
Breaking change: handleActiveChainsUpdate(dataSourceId, activeChains, previousChains) and the onActiveChainsUpdated callback now require previousChains, and all balance data sources were updated/tests adjusted to pass both the source name and previous chain list.

Refactors subscription management to stop tracking per-source chains
in AssetsController and instead assign chains by querying each data source’s getActiveChainsSync() in a fixed priority order; registerDataSources/assignment helpers are removed, unsubscribe logic now operates on data source instances, and docs are updated accordingly. Also fixes RpcDataSource to update its internal/state activeChains before firing the callback so sync reads during resubscribe see the new chains.

^{Written by Cursor
Bugbot for commit 509e716. This will update automatically on new commits. Configure
here.}

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