Fix duplicate HTML content in HtmlMonographFilePlugin

[zielaq]

Summary

• Fix TypeError in HtmlMonographFilePlugin::downloadCallback() when viewing HTML publication format files not assigned to a chapter
• $submissionFile->getData('chapterId') returns null for non-chapter files, causing ChapterDAO::getChapter(int) to throw a TypeError in PHP 8
• Hook::run() silently catches the plugin exception and continues, so the hook returns CONTINUE instead of ABORT — CatalogBookHandler::download() then sends the file a second time via app()->get('file')->download(), resulting in duplicate HTML content

Fix

Guard the getChapter() call with a null check (same pattern already used in CatalogBookHandler::download() at line 420):

// Before
$chapter = $chapterDao->getChapter($submissionFile->getData('chapterId'));

// After
$chapterId = $submissionFile->getData('chapterId');
$chapter = $chapterId ? $chapterDao->getChapter((int) $chapterId) : null;

Test plan

• Upload an HTML file as a publication format without assigning it to a chapter
• Publish the submission and view the HTML file via /catalog/view/{id}/{formatId}/{fileId}
• Verify the HTML content appears only once (not duplicated)
• Upload an HTML file assigned to a chapter and verify it still works correctly

[CLAassistant]

All committers have signed the CLA.