Fix embedded link background in assistant error messages (#12917)

Fixes the issue where links in error messages (e.g., Bedrock
authentication errors) in the Configure Language Model Providers dialog
had a white background in some themes, making them unreadable. Adds
`background-color: transparent` to the `EmbeddedLink` component's anchor
styling.

Author: samclark2015

src/vs/base/browser/ui/positronComponents/embeddedLink/embeddedLink.css

@@ -5,4 +5,5 @@
 
 .embedded-link a {
 	color: var(--vscode-textLink-foreground);
+	background-color: transparent;
 }