How to prevent line breaks in list items using CSS

Use white-space: nowrap;^{[1] [2]} or give that link more space by setting li's width to greater values.

^[1] § 3. White Space and Wrapping: the white-space property - W3 CSS Text Module Level 3
^[2] white-space - CSS: Cascading Style Sheets | MDN

If you want to achieve this selectively (ie: only to that particular link), you can use a non-breaking space instead of a normal space:

<li>submit&nbsp;resume</li>

https://en.wikipedia.org/wiki/Non-breaking_space#Encodings

edit: I understand that this is HTML, not CSS as requested by the OP, but some may find it helpful…

You could add this little snippet of code to add a nice "…" to the ending of the line if the content is to large to fit on one line:

li {
  overflow: hidden; 
  text-overflow: ellipsis;
  white-space: nowrap;
}

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