feat: Improve test coverage and review performance

This commit significantly enhances the test coverage for the `react-use-downloader` hook and related functions.

Key improvements include:

- **`resolver` function:** Added direct unit tests for various scenarios, including successful stream closure, stream errors, invalid responses (not ok, null body), and different content/x-file-size header combinations.
- **`cancel` functionality:** Ensured that ongoing downloads can be correctly cancelled via the `cancel()` method, with appropriate state updates (isInProgress, error).
- **Download timeout:** Added tests to verify that the `timeout` parameter in the `download` function correctly aborts downloads and sets the error state. Tested scenarios both where timeout is exceeded and where download completes before timeout.
- **`jsDownload` edge cases:** Improved tests for `jsDownload` by covering behavior when `tempLink.download` is undefined and ensuring `appendChild` and `removeChild` are correctly called on `document.body`.
- **`setErrorCallback` mapping:** Added tests to verify the error message mapping within the hook, ensuring known errors are translated and unknown errors are passed through.

Additionally, a minor performance review of the `jsDownload` `setTimeout` duration and the `elapsed` state update interval was conducted. No changes were deemed necessary as the current values are standard and appropriate.

These changes increase the robustness of the library by ensuring core functionalities are well-tested and behave as expected under various conditions.

Author: google-labs-jules[bot]