BookBreadcrumbBuilder needs to always set the route.book_navigation cache context
>>> [!note] Migrated issue
<!-- Drupal.org comment -->
<!-- Migrated from issue #3472014. -->
Reported by: [kristiaanvandeneynde](https://www.drupal.org/user/1345130)
Related to !15
>>>
<h3 id="summary-problem-motivation">Problem/Motivation</h3>
<p>We recently fixed breadcrumb builders in core as they were not consistently adding cache contexts based on the logic in their applies() method. Let's fix this in Book too so that you can get rid of the warning on test runs.</p>
<p>See: <span class="drupalorg-gitlab-issue-link project-issue-status-info project-issue-status-7"><a href="https://www.drupal.org/project/drupal/issues/2719721" title="Status: Closed (fixed)">#2719721: BreadcrumbBuilder::applies() mismatch with cacheability metadata</a></span></p>
<h4 id="summary-steps-reproduce">Steps to reproduce</h4>
<p>Watch any test run and spot the warnings.</p>
<h3 id="summary-proposed-resolution">Proposed resolution</h3>
<p>Same fix as core.</p>
<h3 id="summary-remaining-tasks">Remaining tasks</h3>
<p>None, will create MR right away.</p>
<h3 id="summary-ui-changes">User interface changes</h3>
<p>N/A</p>
<h3 id="summary-api-changes">API changes</h3>
<p>N/A</p>
<h3 id="summary-data-model-changes">Data model changes</h3>
<p>N/A</p>
issue