Use string/number signature to get contextual type

Fixes #26587

src/compiler/checker.ts

@@ -16708,6 +16708,8 @@ namespace ts {
                             return restType;
                         }
                     }
+                    return isNumericLiteralName(name) && getIndexTypeOfContextualType(type, IndexKind.Number) ||
+                        getIndexTypeOfContextualType(type, IndexKind.String);
                 }
                 return undefined;
             }, /*noReductions*/ true);