FEATURE: Allow showing hashtag autocomplete results without term (#19219)

This commit allows us to type # in the UI and present autocomplete
results immediately with the following logic for the topic composer,
and reversed for the chat composer:

* Categories the user can access and has not muted sorted by `topic_count`
* Tags the user can access and has not muted sorted by `topic_count`
* Chat channels the user is a member of sorted by `messages_count`

So in effect, we allow searching for hashtags without a search term.
To do this we add a new `search_without_term` to each data source so
each one can define how it wants to handle this logic.

plugins/chat/spec/lib/chat_channel_hashtag_data_source_spec.rb

@@ -5,9 +5,24 @@ RSpec.describe Chat::ChatChannelHashtagDataSource do
   fab!(:category) { Fabricate(:category) }
   fab!(:group) { Fabricate(:group) }
   fab!(:private_category) { Fabricate(:private_category, group: group) }
-  fab!(:channel1) { Fabricate(:chat_channel, slug: "random", name: "Zany Things", chatable: category, description: "Just weird stuff") }
+  fab!(:channel1) do
+    Fabricate(
+      :chat_channel,
+      slug: "random",
+      name: "Zany Things",
+      chatable: category,
+      description: "Just weird stuff",
+      messages_count: 245,
+    )
+  end
   fab!(:channel2) do
-    Fabricate(:chat_channel, slug: "secret", name: "Secret Stuff", chatable: private_category)
+    Fabricate(
+      :chat_channel,
+      slug: "secret",
+      name: "Secret Stuff",
+      chatable: private_category,
+      messages_count: 78,
+    )
   end
   let!(:guardian) { Guardian.new(user) }
 
@@ -109,4 +124,41 @@ RSpec.describe Chat::ChatChannelHashtagDataSource do
       )
     end
   end
+
+  describe "#search_without_term" do
+    fab!(:channel3) { Fabricate(:chat_channel, slug: "general", messages_count: 24) }
+    fab!(:channel4) { Fabricate(:chat_channel, slug: "chat", messages_count: 435) }
+    fab!(:channel5) { Fabricate(:chat_channel, slug: "code-review", messages_count: 334) }
+    fab!(:membership2) do
+      Fabricate(:user_chat_channel_membership, user: user, chat_channel: channel1)
+    end
+    fab!(:membership2) do
+      Fabricate(:user_chat_channel_membership, user: user, chat_channel: channel2)
+    end
+    fab!(:membership3) do
+      Fabricate(:user_chat_channel_membership, user: user, chat_channel: channel3)
+    end
+    fab!(:membership4) do
+      Fabricate(:user_chat_channel_membership, user: user, chat_channel: channel4)
+    end
+    fab!(:membership5) do
+      Fabricate(:user_chat_channel_membership, user: user, chat_channel: channel5)
+    end
+
+    it "returns distinct channels for messages that have been recently created in the past 2 weeks" do
+      expect(described_class.search_without_term(guardian, 5).map(&:slug)).to eq(
+        %w[chat code-review random general],
+      )
+    end
+
+    it "does not return channels the user does not have permission to view" do
+      expect(described_class.search_without_term(guardian, 5).map(&:slug)).not_to include("secret")
+    end
+
+    it "does not return channels where the user is not following the channel via user_chat_channel_memberships" do
+      membership5.destroy
+      membership3.update!(following: false)
+      expect(described_class.search_without_term(guardian, 5).map(&:slug)).to eq(%w[chat random])
+    end
+  end
 end