Assert that autosuggestions are always valid

This documents an invariant established by 532abaddae (Invalidate stale
autosuggestions eagerly, 2024-12-25). It was initially broken but fixed in
ba4ead6ead (Stop saving autosuggestions that we can't restore, 2025-01-17).

src/reader.rs

@@ -4881,7 +4881,13 @@ fn update_autosuggestion(&mut self) {
         }
 
         let el = &self.data.command_line;
+        let autosuggestion = &self.autosuggestion;
         if self.is_at_line_with_autosuggestion() {
+            assert!(string_prefixes_string_maybe_case_insensitive(
+                autosuggestion.icase,
+                &el.text()[autosuggestion.search_string_range.clone()],
+                &autosuggestion.text
+            ));
             return;
         }