fix: Include private channels in Slack integration pagination (#11751)

Fixes
https://linear.app/chatwoot/issue/CW-4507/slack-integration-not-showing-private-channels
  ## Problem

When the Slack workspace has many channels (requiring multiple API
requests to fetch all of them), our system was only looking for private
channels in the first batch of results. All subsequent batches were
missing the instruction to include private channels, so they only
returned public channels.

  ## Root Cause
- Initial API call correctly specified `types:
'public_channel,private_channel'`
- Pagination loop only passed `cursor` parameter, omitting `types` and
`exclude_archived`
  - Subsequent pages defaulted to public channels only

  ## Changes
- Fixed parameter formatting in `types` (removed space:
`'public_channel, private_channel'` →
`'public_channel,private_channel'`)
- Added missing `types` and `exclude_archived` parameters to paginated
`conversations_list` calls

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

lib/integrations/slack/channel_builder.rb

@@ -24,10 +24,14 @@ class Integrations::Slack::ChannelBuilder
   end
 
   def channels
-    conversations_list = slack_client.conversations_list(types: 'public_channel, private_channel', exclude_archived: true)
+    conversations_list = slack_client.conversations_list(types: 'public_channel,private_channel', exclude_archived: true)
     channel_list = conversations_list.channels
     while conversations_list.response_metadata.next_cursor.present?
-      conversations_list = slack_client.conversations_list(cursor: conversations_list.response_metadata.next_cursor)
+      conversations_list = slack_client.conversations_list(
+        cursor: conversations_list.response_metadata.next_cursor,
+        types: 'public_channel,private_channel',
+        exclude_archived: true
+      )
       channel_list.concat(conversations_list.channels)
     end
     channel_list