fix: apply breadcrumbs font-size var to overlay items in base styles (#12085) (CP: 25.2) (#12090)

This PR cherry-picks changes from the original PR #12085 to branch 25.2.

---

#### Original PR description
> ### Motivation
> 
> In base styles, `--vaadin-breadcrumbs-font-size` did not control the
font size of items inside `vaadin-breadcrumbs-overlay`, unlike Lumo and
Aura.
> 
> ### Reproduction
> 
> ```css
> vaadin-breadcrumbs {
>   --vaadin-breadcrumbs-font-size: 12px;
> }
> ```
> 
> The overflow overlay items stayed at the default 16px.
> 
> ### Changes
> 
> Overlay items are slotted into the overlay's shadow content, so they
inherit `font-size` from the overlay content in the flattened tree, not
from the breadcrumbs host. Set `font-size` from the variable directly on
the item host, matching Lumo and Aura. The `1em` fallback resolves to
the inherited value, so rendering is unchanged when the variable is
unset.
> 
> Fixes #12012
> 
> ---
> 
> 🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Sascha Ißbrücker <sissbruecker@vaadin.com>

Author: vaadin-bot

packages/breadcrumbs/src/styles/vaadin-breadcrumbs-item-base-styles.js

@@ -11,6 +11,7 @@ export const breadcrumbsItemStyles = css`
     display: inline-flex;
     align-items: baseline;
     flex-shrink: 0;
+    font-size: var(--vaadin-breadcrumbs-font-size, 1em);
   }
 
   :host([hidden]) {