feat: Add user attribution to Linear integration with actor authorization (#11774)

- Add `actor=app` parameter to Linear OAuth authorization URL for
consistent app-level authorization
https://linear.app/developers/oauth-actor-authorization
- Implement user attribution for Linear issue creation and linking using
`createAsUser` and `displayIconUrl` parameters
- Enhance Linear integration to properly attribute actions to specific
Chatwoot agents

**Note**
- The displayIconUrl parameter is being sent correctly to Linear's
GraphQL API (verified through testing), but there is an issues with icon
is not attaching properly.
- We might need to disconnect the integration connect again.

app/models/integrations/app.rb

@@ -73,7 +73,8 @@ class Integrations::App
       "redirect_uri=#{self.class.linear_integration_url}",
       "state=#{encode_state}",
       'scope=read,write',
-      'prompt=consent'
+      'prompt=consent',
+      'actor=app'
     ].join('&')
   end