test: add test for global style rules in DisplayHtmlTest

This commit adds a test to `DisplayHtmlTest.kt` to verify that the `wrapMessageContent` function correctly adds global CSS style rules. Specifically, it checks for the presence of `word-break: break-word;` and `overflow-wrap: break-word;` rules applied to the `*` selector.

A helper assertion function `containsStyleRulesFor` is also introduced to facilitate checking CSS rules within the HTML output.