BreadcrumbSource::alterComponent() discards merged cache metadata, causing breadcrumb to be cached without proper cache contexts

Problem/Motivation

BreadcrumbSource::alterComponent() attempts to merge the breadcrumb's cache
metadata into the component element, but the merged result is silently discarded.
CacheableMetadata::merge() returns a new cloned object rather than modifying
the receiver in place, so the original $cache variable is never updated.

The buggy code in src/Plugin/UiPatterns/Source/BreadcrumbSource.php:

  $breadcrumbCache = CacheableMetadata::createFromRenderArray($renderable);
  $cache = CacheableMetadata::createFromRenderArray($element);
  $cache->merge($breadcrumbCache); // returns a new object — result discarded
  $cache->applyTo($element);       // $cache is still unmerged; breadcrumb contexts never applied
  

Because the merged clone is discarded, none of the breadcrumb builder's cache contexts
(e.g. url.path, route, or any custom contexts) are propagated
to the component element. The rendered breadcrumb block is therefore cached without the
correct cache contexts, and the first breadcrumb generated for a given block placement is
served for all subsequent requests that share the same insufficient cache key — in
practice, the same wrong breadcrumb appears across unrelated pages.

Steps to reproduce

1. Place a UI Patterns breadcrumb block using source_id: breadcrumb.
2. Implement a custom BreadcrumbBuilderInterface that adds cache contexts such as url.path.
3. Visit two different pages. Both pages will display the breadcrumb generated for the first page loaded, regardless of the current URL.

Proposed resolution

Chain applyTo() on the return value of merge() so the merged
metadata is actually applied to the element:

  $breadcrumbCache = CacheableMetadata::createFromRenderArray($renderable);
  $cache = CacheableMetadata::createFromRenderArray($element);
  $cache->merge($breadcrumbCache)->applyTo($element);
  

Remaining tasks

Write a test confirming that cache contexts from the breadcrumb builder are present on the rendered component element.

User interface changes

None.

API changes

None.

Data model changes

None.