## Explanation

The current implementation of `TokensController` relies heavily on a
single selected network (`chainId`) as an instance property, which makes
multi-chain token handling brittle and hard to scale. Additionally, some
token operations unintentionally assume a persistent network context,
introducing potential bugs when switching networks or performing actions
concurrently across different chains.

This PR refactors the `TokensController` to eliminate reliance on an
internal `#chainId` property. Instead, all methods that require a
network context now accept a `networkClientId` as an explicit parameter.
This improves the controller’s ability to operate safely and predictably
in a multi-chain environment.

The `#onNetworkDidChange` logic and associated subscription were removed
since network changes are now handled at the method level with explicit
`networkClientId` parameters. All relevant internal state mutations and
token metadata fetches are scoped to the appropriate chain via the
`networkClientId -> chainId` mapping.

This refactor is part of a broader initiative to support simultaneous
multi-chain state handling across the codebase (see recent changes to
`TokenRatesController` and `AccountTrackerController`).

integration with UI:
extension: MetaMask/metamask-extension#32274


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## References
* Fixes #5580 
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## Changelog

### `@metamask/assets-controllers`

- **UPDATE:** Refactor `TokensController` to support handling multiple
chains in parallel. All network-dependent methods now require an
explicit `networkClientId` parameter, enabling token operations to be
fully scoped to a specific chain. This improves reliability and
predictability when managing tokens across different networks.
- **BREAKING:** Remove internal reliance on a single `chainId` instance
property in `TokensController`. All chain-specific logic has been
externalized. Consumers must now explicitly pass a `networkClientId` to
methods that interact with the token state. This change may require
updates on the client side to ensure correct chain context is provided
during token actions.


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## 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](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
- [ ] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes