Fix the comment button color of newly created highlights in the annotation layer

[LoukasPap]

Fixes #22087

When a highlight with a comment is created and the highlight tool is closed, the comment button rendered in the annotation layer was gray instead of using the highlight's color. PopupElement.#updateColor returned early for any element with an extra popup element, which is always the case for the editor-backed annotation created for a new highlight, so no background-color was ever set. The update is now only skipped when there is no associated editor, mirroring the check already done in #updateCommentButtonPosition.

An integration test is added in test/integration/comment_spec.mjs.

Test cases

Test 1: The comment button keeps the highlight color after closing the highlight tool

1. Open any PDF with text, with comments enabled.
2. Select some text and click the comment button of the floating toolbar.
3. Type a comment and save it.
4. Select the red highlight color.
5. Close the highlight tool.
6. The comment button is red (it was gray before).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.27%. Comparing base (17bb244) to head (1e4d6fc).
⚠️ Report is 27 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #22089      +/-   ##
==========================================
- Coverage   90.30%   90.27%   -0.03%     
==========================================
  Files         269      269              
  Lines       67362    67362              
==========================================
- Hits        60828    60813      -15     
- Misses       6534     6549      +15     

Flag	Coverage Δ
browsertest	66.06% <0.00%> (-0.01%)	⬇️
fonttest	8.95% <ø> (ø)
integrationtest	69.22% <100.00%> (-0.02%)	⬇️
unittest	59.86% <0.00%> (-0.02%)	⬇️
unittestcli	58.39% <0.00%> (-0.02%)	⬇️

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