DEV: Add proper error response when searching with an invalid page param (#31026)

Previously, for a search query with `page=11` or higher, we were quietly
returning the page 10 results. The frontend app isn't affected because
it sets its own limit to 10 pages, but still, this response from the
search endpoint does not make sense.

This change switches to returning a 400 error when the `page` parameter
is above the allowed limit (a max of 10).

app/controllers/search_controller.rb

@@ -5,6 +5,8 @@ class SearchController < ApplicationController
   skip_before_action :check_xhr, only: :show
   after_action :add_noindex_header
 
+  PAGE_LIMIT = 10
+
   def self.valid_context_types
     %w[user topic category private_messages tag]
   end
@@ -28,6 +30,9 @@ class SearchController < ApplicationController
     page = permitted_params[:page]
     # check for a malformed page parameter
     raise Discourse::InvalidParameters if page && (!page.is_a?(String) || page.to_i.to_s != page)
+    if page && page.to_i > PAGE_LIMIT
+      raise Discourse::InvalidParameters.new("page parameter must not be greater than 10")
+    end
 
     discourse_expires_in 1.minute
 
@@ -35,7 +40,7 @@ class SearchController < ApplicationController
       type_filter: "topic",
       guardian: guardian,
       blurb_length: 300,
-      page: ([page.to_i, 1].max if page.to_i <= 10),
+      page: [page.to_i, 1].max,
     }
 
     context, type = lookup_search_context