feat: clean up new controller

[salimtb]

Explanation

This PR adds comprehensive unit tests for the new @metamask/assets-controller package to ensure code quality and maintainability.

Current state: The assets-controller package was recently introduced with a middleware-based architecture for unified asset management across all blockchain networks. However, test coverage was limited, leaving critical code paths untested.

Changes in this PR:

• Added unit tests for all data sources:
  • AccountsApiDataSource (18 tests, includes timer cleanup fix using .unref() to prevent Jest worker process hanging)
  • BackendWebsocketDataSource (19 tests)
  • PriceDataSource (29 tests)
  • TokenDataSource (20 tests)
  • SnapDataSource (47 tests)
• Added unit tests for DetectionMiddleware (13 tests, 100% coverage)
• Expanded unit tests for AssetsController (42 tests, coverage increased from 33% to 81%)

Test architecture patterns used:

• setupController / withController helper functions for consistent test setup
• Proper messenger mocking with MOCK_ANY_NAMESPACE
• Test cleanup to prevent resource leaks
• it.each for parameterized tests where applicable
• Flat test structure avoiding deeply nested describe blocks

References

• Related to feat(assets-controller): first version of AssetsController #7685 (AssetsController middleware architecture)
• Related to chore: create new assets controller package #7587 (Initial assets-controller release)

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

Significantly increases test coverage and validates core behaviors across the new middleware-based assets stack.

• Add extensive unit tests for AssetsController, DetectionMiddleware, and data sources: AbstractDataSource, AccountsApiDataSource, BackendWebsocketDataSource, PriceDataSource, RpcDataSource, and SnapDataSource
• Cover subscriptions, middleware chaining, lifecycle/events (app/keyring/network), WebSocket handling, pricing polls, and snap-based balances
• Update RpcDataSource to import BalanceFetcher, MulticallClient, and TokenDetector from evm-rpc-services (was rpc-datasource)
• Update CHANGELOG.md to note test additions

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