DEV: Support posts-min:<count> and posts-max:<count> on /filter (#21090)

This commit adds support for the `posts-min:<count>` and
`posts-max:<count>` filters for the topics filtering query language.
`posts-min:1` will filter for topics with at least a one post while
`posts-max:3` will filter foor topics with a maximum of 3 posts.

If the filter has an invalid value, i.e string that cannot be converted
into an integer, the filter will be ignored.

If either of each filter is specify multiple times, only the last
occurence of each filter will be taken into consideration.

spec/lib/topics_filter_spec.rb

@@ -846,5 +846,88 @@ RSpec.describe TopicsFilter do
         end
       end
     end
+
+    describe "when filtering by number of posts in a topic" do
+      fab!(:topic_with_1_post) { Fabricate(:topic, posts_count: 1) }
+      fab!(:topic_with_2_posts) { Fabricate(:topic, posts_count: 2) }
+      fab!(:topic_with_3_posts) { Fabricate(:topic, posts_count: 3) }
+
+      describe "when query string is `posts-min:1`" do
+        it "should only return topics with at least 1 post" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posts-min:1")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_1_post.id, topic_with_2_posts.id, topic_with_3_posts.id)
+        end
+      end
+
+      describe "when query string is `posts-min:3`" do
+        it "should only return topics with at least 3 posts" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posts-min:3")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_3_posts.id)
+        end
+      end
+
+      describe "when query string is `posts-max:1`" do
+        it "should only return topics with at most 1 post" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posts-max:1")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_1_post.id)
+        end
+      end
+
+      describe "when query string is `posts-max:3`" do
+        it "should only return topics with at most 3 posts" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posts-max:3")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_1_post.id, topic_with_2_posts.id, topic_with_3_posts.id)
+        end
+      end
+
+      describe "when query string is `posts-min:1 posts-max:2`" do
+        it "should only return topics with at least a post and at most 2 posts" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posts-min:1 posts-max:2")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_1_post.id, topic_with_2_posts.id)
+        end
+      end
+
+      describe "when query string is `posts-min:3 posts-min:2 posts-max:1 posts-max:3`" do
+        it "should only return topics with at least 2 posts and at most 3 posts as it ignores earlier filters which are duplicated" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posts-min:3 posts-min:2 posts-max:1 posts-max:3")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_2_posts.id, topic_with_3_posts.id)
+        end
+      end
+
+      describe "when query string is `posts-min:invalid posts-max:invalid`" do
+        it "should ignore the filters with invalid values" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posts-min:invalid posts-max:invalid")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_1_post.id, topic_with_2_posts.id, topic_with_3_posts.id)
+        end
+      end
+    end
   end
 end