feat: Add conversations summary CSV export (#13110)

# Pull Request Template

## Description

This PR adds support for exporting conversation summary reports as CSV.
Previously, the Conversations report incorrectly showed an option to
download agent reports; this has now been fixed to export
conversation-level data instead.

Fixes
https://linear.app/chatwoot/issue/CW-6176/conversation-reports-export-button-exports-agent-reports-instead

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

### Screenshot
<img width="1859" height="1154" alt="image"
src="https://github.com/user-attachments/assets/419d26f4-fda9-4782-aea6-55ffad0c37ab"
/>



## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

config/locales/en.yml

@@ -172,6 +172,14 @@ en:
       avg_resolution_time: Avg resolution time
       resolution_count: Resolution Count
       avg_customer_waiting_time: Avg customer waiting time
+    conversation_csv:
+      conversations_count: Conversations
+      incoming_messages_count: Messages received
+      outgoing_messages_count: Messages sent
+      avg_first_response_time: Avg first response time
+      avg_resolution_time: Avg resolution time
+      resolution_count: Resolution count
+      avg_customer_waiting_time: Avg customer waiting time
     conversation_traffic_csv:
       timezone: Timezone
     sla_csv:

config/routes.rb

@@ -430,6 +430,7 @@ Rails.application.routes.draw do
               get :labels
               get :teams
               get :conversations
+              get :conversations_summary
               get :conversation_traffic
               get :bot_metrics
             end