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>

app/views/api/v2/accounts/reports/conversations_summary.csv.erb

@@ -0,0 +1,16 @@
+<%= CSVSafe.generate_line [I18n.t('reports.period', since: Date.strptime(params[:since], '%s'), until: Date.strptime(params[:until], '%s'))] %>
+
+<% headers = [
+  I18n.t('reports.conversation_csv.conversations_count'),
+  I18n.t('reports.conversation_csv.incoming_messages_count'),
+  I18n.t('reports.conversation_csv.outgoing_messages_count'),
+  I18n.t('reports.conversation_csv.avg_first_response_time'),
+  I18n.t('reports.conversation_csv.avg_resolution_time'),
+  I18n.t('reports.conversation_csv.resolution_count'),
+  I18n.t('reports.conversation_csv.avg_customer_waiting_time')
+]
+%>
+<%= CSVSafe.generate_line headers -%>
+<% @report_data.each do |row| %>
+<%= CSVSafe.generate_line row -%>
+<% end %>