Convert existing '## Sample code' samples to '{@tool sample}...{@end-tool}' form. (#24077)

This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.
diff --git a/dev/snippets/config/skeletons/application.html b/dev/snippets/config/skeletons/application.html
index 347b6e1..afe9c4c 100644
--- a/dev/snippets/config/skeletons/application.html
+++ b/dev/snippets/config/skeletons/application.html
@@ -5,11 +5,7 @@
 </div>
 <div class="snippet-container">
   <div class="snippet" id="shortSnippet">
-    <div class="snippet-description">
-      {@end-inject-html}
-      {{description}}
-      {@inject-html}
-    </div>
+    {{description}}
     <div class="copyable-container">
       <button class="copy-button-overlay copy-button" title="Copy to clipboard"
               onclick="copyTextToClipboard();">