feat: Add support for grouped file uploads in Slack (#12454)

Fixes
https://linear.app/chatwoot/issue/CW-5646/add-support-for-grouped-file-uploads-in-slack

Previously, when sending multiple attachments to Slack, we uploaded them
one by one. For example, sending 5 images would result in 5 separate
Slack messages. This created clutter and a poor user experience, since
Slack displayed each file as an individual message.
This PR updates the implementation to group all attachments from a
message and send them as a single Slack message. As a result,
attachments now appear together in one grouped block, providing a much
cleaner and more intuitive experience for users.

**Before:** 
Each file uploaded as a separate Slack message.
<img width="400" height="800" alt="before"
src="https://github.com/user-attachments/assets/c8c7f666-549b-428f-bd19-c94e39ed2513"
/>

**After:** 
All files from a single message grouped and displayed together in one
Slack message (similar to how Slack natively handles grouped uploads).
<img width="400" height="800" alt="after"
src="https://github.com/user-attachments/assets/0b1f22d5-4d37-4b84-905a-15e742317e72"
/>

**Changes**

- Upgraded Slack file upload implementation to use the new multiple
attachments API available in slack-ruby-client `v2.7.0`.
- Updated attachment handling to upload all files from a message in a
single API call.
- Enabled proper attachment grouping in Slack, ensuring related files
are presented together.

Gemfile.lock

@@ -292,7 +292,7 @@ GEM
       logger
     faraday-follow_redirects (0.3.0)
       faraday (>= 1, < 3)
-    faraday-mashify (0.1.1)
+    faraday-mashify (1.0.0)
       faraday (~> 2.0)
       hashie
     faraday-multipart (1.0.4)
@@ -876,8 +876,8 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.13.2)
     simplecov_json_formatter (0.1.4)
-    slack-ruby-client (2.5.2)
-      faraday (>= 2.0)
+    slack-ruby-client (2.7.0)
+      faraday (>= 2.0.1)
       faraday-mashify
       faraday-multipart
       gli
@@ -1103,7 +1103,7 @@ DEPENDENCIES
   sidekiq_alive
   simplecov (>= 0.21)
   simplecov_json_formatter
-  slack-ruby-client (~> 2.5.2)
+  slack-ruby-client (~> 2.7.0)
   spring
   spring-watcher-listen
   squasher