Fix RedHerbSidebarHook calling removed ViewHtmlBuilder method

`ViewHtmlBuilder::pageHasMainSubject()` was removed in #795 and the
predicate moved to `PageSubjectsLookup`. The internal call site in
`NeoWikiHooks` was updated, but `RedHerbSidebarHook` still referenced
the removed method via the default closure in its constructor, which
fired on every `SidebarBeforeOutput` invocation against an existing
page, producing an HTTP 500.

Switch the default closure to `PageSubjectsLookup::pageHasMainSubject()`,
matching the call shape now used in `NeoWikiHooks::getNeoWikiAppHtml()`.
The closure signature stays `Title -> bool`; the conversion to `PageId`
is internal so existing test stubs need no changes.

Follows-up to #795

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: malberts

tests/RedHerb/src/RedHerbSidebarHook.php

@@ -7,6 +7,7 @@
 use Closure;
 use MediaWiki\Hook\SidebarBeforeOutputHook;
 use MediaWiki\Title\Title;
+use ProfessionalWiki\NeoWiki\Domain\Page\PageId;
 use ProfessionalWiki\NeoWiki\NeoWikiExtension;
 use Skin;
 
@@ -16,7 +17,8 @@ class RedHerbSidebarHook implements SidebarBeforeOutputHook {
 
 	public function __construct( ?Closure $pageHasMainSubject = null ) {
 		$this->pageHasMainSubject = $pageHasMainSubject ?? static fn ( Title $title ): bool =>
-			NeoWikiExtension::getInstance()->newViewHtmlBuilder()->pageHasMainSubject( $title );
+			NeoWikiExtension::getInstance()->newPageSubjectsLookup()
+				->pageHasMainSubject( new PageId( $title->getArticleID() ) );
 	}
 
 	public function onSidebarBeforeOutput( $skin, &$sidebar ): void {