Tests the formatter output

I tested this MR locally on a clean Drupal installation and went through all of the scenarios covered by the new kernel test.

I also verified the formatter manually with different settings, including:

• * Default formatter output
• * Trimming with and without ellipsis
• * HTML filtering with allowed tags
• * Auto paragraph handling
• * Using the summary instead of the full text
• * Trimming the summary

In each case, the rendered output matched the expected result, and the new kernel test passed successfully.

I also ran the module's test suite after applying the MR and didn't encounter any regressions or unexpected failures.

Overall, this is a nice improvement. The data provider makes the test easy to read and extend, and it provides good coverage for the formatter's core functionality. This should help catch regressions if changes are made to the formatter in the future.

I only have a couple of minor suggestions:

• * It would be good to add return type hints (: void for the test method and : array for the data provider) to keep the test consistent with current Drupal coding standards.
• * If token replacement is supported by the formatter, it may also be worth adding a dedicated test case for that setting in a follow-up issue.

Other than those minor suggestions, the implementation looks good to me, and all the scenarios I tested locally are working as expected.