Incomplete test coverage makes coverage threshold messages difficult to resolve

Nobody has claimed this yet.

There are several packages in this repo that do not have 100% test coverage. In fact we've explicitly set coverage thresholds to match the true coverage today. This makes sense on the surface, but occasionally, a change is made in a PR which causes the coverage to slip below the threshold, which causes a failed build (example: https://github.com/MetaMask/core/actions/runs/9473788927/job/26102151727?pr=4254). These errors are difficult to debug because it is unclear just by looking at the build output where the lack of coverage lies. We need to run `yarn test:verbose` locally on `develop` (or the base branch) and then again against the topic branch and manually compare the output visually. This is a pain. It would be great if we didn't need to do this.

## Acceptance Criteria

We are able to bring all of the numbers underneath the `coverageThreshold` portion of the Jest configuration to 100% for the following packages:

- `@metamask/assets-controllers`
- `@metamask/base-controller`
- `@metamask/controller-utils`
- `@metamask/gas-fee-controller`
- `@metamask/json-rpc-middleware-stream`
- `@metamask/keyring-controller`
- `@metamask/network-controller`
- `@metamask/polling-controller`
- `@metamask/preferences-controller`
- `@metamask/profile-sync-controller`
- `@metamask/rate-limit-controller`
- `@metamask/transaction-controller`

Start with the Jest configuration's coverageThreshold section and run `yarn test:verbose` on `develop`, then on a topic branch, to identify uncovered code. Work through the twelve listed packages and add the missing test coverage. Done means every listed package has all coverage numbers at 100% and the coverage checks pass.

Written by the indexing model from the issue text.