FIX: TopicSummarization workaround for Postgres' discrete range types (#23105)

Our code assumed the content_range interval was inclusive, but they are open-ended due to Postgres' [discrete range types](https://www.postgresql.org/docs/current/rangetypes.html#RANGETYPES-DISCRETE), meaning [1,2] will be represented as [1,3).

It also fixes some flaky tests due to test data not being correctly setup and the registry not being resetted after each test.

spec/lib/summarization/base_spec.rb

@@ -10,11 +10,13 @@ describe Summarization::Base do
   before do
     group.add(user)
 
-    strategy = DummyCustomSummarization.new("dummy")
+    strategy = DummyCustomSummarization.new({ summary: "dummy", chunks: [] })
     plugin.register_summarization_strategy(strategy)
     SiteSetting.summarization_strategy = strategy.model
   end
 
+  after { DiscoursePluginRegistry.reset_register!(:summarization_strategies) }
+
   describe "#can_see_summary?" do
     context "when the user cannot generate a summary" do
       before { SiteSetting.custom_summarization_allowed_groups = "" }