Remove current chain from addTokens function

tokenController:
The `addToken` method currently relies on a fallback to a default `chainId` and includes a TODO note to remove this behavior once the method is fully parameterized by `chainId` (via `networkClientId`). Additionally, the token metadata fetch is not yet parameterized by `networkClientId`.

Start by locating tokenController and reading the addToken method, especially its chainId fallback and TODO. Trace how networkClientId is passed into the method and how token metadata is fetched. Done means removing the current-chain fallback and parameterizing both token handling and metadata fetching by networkClientId.

Written by the indexing model from the issue text.