FIX: Fetch categories data using specific endpoint (#543)

It used to fetch it from /site.json, but /categories.json is the more
appropriate one. This one also implements pagination, so we have to do
one request per page.

lib/ai_bot/tools/discourse_meta_search.rb

@@ -142,14 +142,26 @@ module DiscourseAi
 
         def self.categories
           return @categories if defined?(@categories)
-          url = "https://meta.discourse.org/site.json"
-          json = JSON.parse(Net::HTTP.get(URI(url)))
-          @categories =
-            json["categories"]
-              .map do |c|
-                [c["id"], { "name" => c["name"], "parent_category_id" => c["parent_category_id"] }]
-              end
-              .to_h
+
+          @categories = {}
+
+          page = 0
+          loop do
+            page += 1
+            url = "https://meta.discourse.org/categories.json?page=#{page}"
+
+            json = JSON.parse(Net::HTTP.get(URI(url)))
+            break if json["category_list"]["categories"].blank?
+
+            json["category_list"]["categories"].each do |c|
+              @categories[c["id"]] = {
+                "name" => c["name"],
+                "parent_category_id" => c["parent_category_id"],
+              }
+            end
+          end
+
+          @categories
         end
 
         def description_args

spec/fixtures/search_meta/categories_page_2.json

@@ -0,0 +1,7 @@
+{
+  "category_list": {
+    "can_create_category": false,
+    "can_create_topic": false,
+    "categories": []
+  }
+}

spec/lib/modules/ai_bot/tools/discourse_meta_search_spec.rb

@@ -11,12 +11,27 @@ RSpec.describe DiscourseAi::AiBot::Tools::DiscourseMetaSearch do
 
   let(:mock_search_json) { plugin_file_from_fixtures("search.json", "search_meta").read }
 
-  let(:mock_site_json) { plugin_file_from_fixtures("site.json", "search_meta").read }
+  let(:mock_categories_page_1) do
+    plugin_file_from_fixtures("categories_page_1.json", "search_meta").read
+  end
+
+  let(:mock_categories_page_2) do
+    plugin_file_from_fixtures("categories_page_2.json", "search_meta").read
+  end
 
   before do
-    stub_request(:get, "https://meta.discourse.org/site.json").to_return(
+    stub_request(:get, "https://meta.discourse.org/categories.json?page=1").to_return(
       status: 200,
-      body: mock_site_json,
+      body: mock_categories_page_1,
+      headers: {
+      },
+    )
+  end
+
+  before do
+    stub_request(:get, "https://meta.discourse.org/categories.json?page=2").to_return(
+      status: 200,
+      body: mock_categories_page_2,
       headers: {
       },
     )