Make starts-with? and ends-with? tolerate non-string args (return false)

Previously these primitives threw Eval_error if either arg was non-string.
Now they return false, preventing crashes when DOM attributes return nil
values during element processing (e.g. htmx-boot-subtree! iterating
elements with undefined attribute names).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_primitives.ml

@@ -346,13 +346,13 @@ let () =
     | [String s; String prefix] ->
       Bool (String.length s >= String.length prefix &&
             String.sub s 0 (String.length prefix) = prefix)
-    | _ -> raise (Eval_error "starts-with?: 2 string args"));
+    | _ -> Bool false);
   register "ends-with?" (fun args ->
     match args with
     | [String s; String suffix] ->
       let sl = String.length s and xl = String.length suffix in
       Bool (sl >= xl && String.sub s (sl - xl) xl = suffix)
-    | _ -> raise (Eval_error "ends-with?: 2 string args"));
+    | _ -> Bool false);
   register "index-of" (fun args ->
     match args with
     | [String haystack; String needle] ->