qet.addElement: report an unreadable symbol instead of a collision

A symbol file that exists but cannot be read has a null uuid, which the
check against the copy already embedded reported as "would collide with a
different element". Say that the file could not be read, with its full
path and length, since a long path is the usual cause on Windows.

tst_unreadableelement makes the file unreadable through its permissions
and fails on master with the old message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/scripting/qetscriptapi.cpp

@@ -86,6 +86,7 @@
 #include <QSqlError>
 #include <QSqlQuery>
 #include <QSqlRecord>
+#include <QDir>
 #include <QDomDocument>
 #include <QFileInfo>
 #include <QFont>
@@ -677,6 +678,17 @@ QString QetScriptApi::addElement(int folioIndex, const QString &locationPath, do
 		const QString import_path = location.isFileSystem()
 				? QStringLiteral("import/") + location.collectionPath(false)
 				: location.collectionPath(false);
+		// An element file that exists but cannot be read gives a null
+		// uuid(), which the collision check below would misreport as "a
+		// different element" -- say what actually went wrong instead.
+		if (location.isFileSystem()
+				&& location.pugiXml().document_element().empty()) {
+			const QString file = QDir::toNativeSeparators(
+						QFileInfo(location.fileSystemPath()).absoluteFilePath());
+			log(QStringLiteral("qet.addElement: could not read element '%1' (file '%2', "
+								"%3 characters)").arg(locationPath, file).arg(file.size()));
+			return QString();
+		}
 		const ElementsLocation existing(import_path, m_project);
 		if (existing.exist() && existing.uuid() != location.uuid()) {
 			log(QStringLiteral("qet.addElement: '%1' would collide with a different element "