Add debug response for file reading in readFiles method and improve error handling for file accessibility

app/Http/Controllers/SapFileController.php

@@ -129,6 +129,20 @@ class SapFileController extends Controller
             return response()->json(['status' => 'ERROR', 'message' => 'File is empty'], 400);
         }
 
+        // Debug: return first 5 lines to check reading
+        return response()->json([
+            'status' => 'DEBUG_LINES',
+            'file' => $fileName,
+            'lines_sample' => array_slice($lines, 0, 5) // first 5 lines
+        ]);
+
+        if (!file_exists($filePath) || !is_readable($filePath)) {
+            return response()->json([
+                'status' => 'ERROR',
+                'message' => "File {$fileName} not found or not readable"
+            ], 500);
+        }
+
         $table = 'class_characteristics';
         $columns = Schema::getColumnListing($table); // returns lowercase column names