fix: Incorrect contact access in conversations listing (#11797)

# Pull Request Template

## Description

This PR fixes the incorrect contact access in conversations listing API.
Cause:

- `undefined method 'conversations' for nil` error because `@contact` is
not initialized

Solution:
- Using `@contact_inbox` to access `@contact`
- `@contact_inbox` is properly set in the parent controller's
`set_contact_inbox` method

Fixes
https://linear.app/chatwoot/issue/CW-4185/incorrect-contact-access-pattern-in

## Type of change

Please delete options that are not relevant.

- [x] Bug fix (non-breaking change which fixes an issue)

## 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
- [ ] 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/controllers/public/api/v1/inboxes/conversations_controller.rb

@@ -3,7 +3,7 @@ class Public::Api::V1::Inboxes::ConversationsController < Public::Api::V1::Inbox
   before_action :set_conversation, only: [:toggle_typing, :update_last_seen, :show, :toggle_status]
 
   def index
-    @conversations = @contact_inbox.hmac_verified? ? @contact.conversations : @contact_inbox.conversations
+    @conversations = @contact_inbox.hmac_verified? ? @contact_inbox.contact.conversations : @contact_inbox.conversations
   end
 
   def show; end