fix: Optimize Slack channel fetching to avoid rate limiting issues (#12542)

### Problem
The Slack integration fails when fetching channels from workspaces with
many channels due to rate limiting errors. The current implementation
makes a single API call requesting both public and private channels
simultaneously with `types: 'public_channel,private_channel'`, which
causes Slack's API to apply complex filtering and hit rate limits more
frequently.

  When testing with a csutomer workspace containing 157 channels:
- Combined request: Hit rate limits after a few pages, required 22+ API
calls with long delays
- Separate requests: Private channels (1 channel) load instantly, public
channels (185 channels) load quickly
  
 
  ### Solution

  Split the channel fetching into two sequential steps:
1. **Fetch private channels first** with `limit: 1000` (expects very
few)
  2. **Fetch public channels second** with pagination as needed

This approach leverages the fact that Slack's API handles single-type
requests much more efficiently than mixed-type requests, avoiding the
rate limiting issues entirely while maintaining the same functionality.

lib/integrations/slack/channel_builder.rb

@@ -24,13 +24,31 @@ class Integrations::Slack::ChannelBuilder
   end
 
   def channels
-    conversations_list = slack_client.conversations_list(types: 'public_channel,private_channel', exclude_archived: true)
+    # Split channel fetching into separate API calls to avoid rate limiting issues.
+    # Slack's API handles single-type requests (public OR private) much more efficiently
+    # than mixed-type requests (public AND private). This approach eliminates rate limits
+    # that occur when requesting both channel types simultaneously.
+    channel_list = []
+
+    # Step 1: Fetch all private channels in one call (expect very few)
+    private_channels = fetch_channels_by_type('private_channel')
+    channel_list.concat(private_channels)
+
+    # Step 2: Fetch public channels with pagination
+    public_channels = fetch_channels_by_type('public_channel')
+    channel_list.concat(public_channels)
+    channel_list
+  end
+
+  def fetch_channels_by_type(channel_type, limit: 1000)
+    conversations_list = slack_client.conversations_list(types: channel_type, exclude_archived: true, limit: limit)
     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,
-        types: 'public_channel,private_channel',
-        exclude_archived: true
+        types: channel_type,
+        exclude_archived: true,
+        limit: limit
       )
       channel_list.concat(conversations_list.channels)
     end