fix(material/tabs): default stretchTabs value not picked up by nav bar (

#27212)

Fixes that the tab nav bar wasn't picking up the default value for `stretchTabs` from the config.

Fixes #27211.

(cherry picked from commit 88d3906)

src/material/tabs/tab-nav-bar/tab-nav-bar.ts

@@ -373,6 +373,8 @@ export class MatTabNav extends _MatTabNavBase implements AfterContentInit, After
       defaultConfig && defaultConfig.fitInkBarToContent != null
         ? defaultConfig.fitInkBarToContent
         : false;
+    this.stretchTabs =
+      defaultConfig && defaultConfig.stretchTabs != null ? defaultConfig.stretchTabs : true;
   }
 
   override ngAfterContentInit() {