Harden reload-time label/ref recheck against stale document pointers

[Copilot]

Reloads after external file changes could traverse document lists containing stale/invalid LatexDocument*, leading to crashes while calling labelItems() during structure/reference refresh. This change adds pointer-validity gating in the reload/recheck paths.

• Root cause containment: stale pointer filtering

  • Added a local helper to derive the current active-document set from LatexDocuments.
  • Before invoking labelItems() / recheckRefsLabels(), skip null pointers and pointers not present in the active set.

• Applied in all relevant traversal paths

  • LatexDocument::setMasterDocument(..., recheck=true)
  • LatexDocument::recheckRefsLabels(...) (auto list/item aggregation branch)
  • LatexDocuments::addDocsToLoad(...) (post-load syntax/ref refresh)

• Implementation notes

  • Consolidated duplicated validation logic via availableDocumentSet(...).
  • Switched affected loops to explicit range-based iteration for clearer pointer filtering semantics.

const QSet<LatexDocument *> availableDocs = availableDocumentSet(parent);
for (LatexDocument *elem : qAsConst(listOfDocs)) {
    if (!elem || !availableDocs.contains(elem)) continue;
    items << elem->labelItems();
}

• Fixes Crash when reloading files after change by another program. #4563