FIX: add server-side limit for DM channels (#30300)

When fetching DM channels, we would fetch all the user's DM channels,
and limit the number of DM channels we display on the client-side using
the `DIRECT_MESSAGE_CHANNELS_LIMIT`.

This is obviously not scalable as users are having more and more
discussions with other users.

This commit ensures we always limit the number of DM channels we send to the
client.

It adds the `MAX_DM_CHANNEL_RESULTS` in `ChannelFetcher` and use it to
limit the records returned by the server. This value should always be
lower than `DIRECT_MESSAGE_CHANNELS_LIMIT` on the client-side, so that
when the user has more "unread DMs" they still show up on the client as
they read them.

While adding a spec, I also updated the spec that ensures we have a
limit on public channels to use `stub_const` to temporarily lower the number
of fixtures we create during the test.

plugins/chat/lib/chat/channel_fetcher.rb

@@ -4,6 +4,9 @@ module Chat
   class ChannelFetcher
     MAX_PUBLIC_CHANNEL_RESULTS = 50
 
+    # NOTE: this should always be > than `DIRECT_MESSAGE_CHANNELS_LIMIT` in `chat-channel-manager.js`
+    MAX_DM_CHANNEL_RESULTS = 75
+
     def self.structured(guardian, include_threads: false)
       memberships = Chat::ChannelMembershipManager.all_for_user(guardian.user)
       public_channels = secured_public_channels(guardian, status: :open, following: true)
@@ -232,16 +235,19 @@ module Chat
       end
 
       query =
-        query.order("last_message.created_at DESC NULLS LAST").group(
-          "chat_channels.id",
-          "last_message.id",
-        )
+        query
+          .limit(MAX_DM_CHANNEL_RESULTS)
+          .order("last_message.created_at DESC NULLS LAST")
+          .group("chat_channels.id", "last_message.id")
 
       channels = query.to_a
+
       preload_fields =
         User.allowed_user_custom_fields(guardian) +
           UserField.all.pluck(:id).map { |fid| "#{User::USER_FIELD_PREFIX}#{fid}" }
-      User.preload_custom_fields(channels.map { |c| c.chatable.users }.flatten, preload_fields)
+
+      User.preload_custom_fields(channels.flat_map { _1.chatable.users }, preload_fields)
+
       channels
     end