Use Sanitizer::stripAllTags() when generating notification snippets

It adds white-space between block tags and strips invisible tags.

It may be slightly slower (it takes HTML as input rather than DOM, so
we need to serialize the HTML first and then call it, rather than only
find and concatenate text nodes), but the difference is negligible,
and it seems better to use this method than to try to re-implement it.

Test runtime went from ~9.0s to ~9.5s locally, when testing using:
  php tests/phpunit/phpunit.php \
  extensions/DiscussionTools/tests/phpunit/ThreadItemTest.php \
  --filter getText

Bug: T219138
Change-Id: I0cb89ebd2160e1ef499b78573c6688f493a4c42f

includes/CommentItem.php

@@ -4,6 +4,7 @@ namespace MediaWiki\Extension\DiscussionTools;
 
 use DateTimeImmutable;
 use MWException;
+use Sanitizer;
 use Title;
 use Wikimedia\Parsoid\DOM\DocumentFragment;
 use Wikimedia\Parsoid\DOM\Text;
@@ -109,8 +110,8 @@ class CommentItem extends ThreadItem {
 	 * @return string Text
 	 */
 	public function getBodyText( bool $stripTrailingSeparator = false ): string {
-		$fragment = $this->getBodyFragment( $stripTrailingSeparator );
-		return $fragment->textContent ?? '';
+		$html = $this->getBodyHTML( $stripTrailingSeparator );
+		return Sanitizer::stripAllTags( $html );
 	}
 
 	/**

includes/ThreadItem.php

@@ -4,6 +4,7 @@ namespace MediaWiki\Extension\DiscussionTools;
 
 use JsonSerializable;
 use LogicException;
+use Sanitizer;
 use Title;
 use Wikimedia\Parsoid\DOM\Element;
 use Wikimedia\Parsoid\DOM\Node;
@@ -281,8 +282,8 @@ abstract class ThreadItem implements JsonSerializable {
 	 * @return string Text
 	 */
 	public function getText(): string {
-		$fragment = $this->getRange()->cloneContents();
-		return $fragment->textContent ?? '';
+		$html = $this->getHTML();
+		return Sanitizer::stripAllTags( $html );
 	}
 
 	/**