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.

lib/integrations/linear/processor_service.rb

@@ -22,8 +22,8 @@ class Integrations::Linear::ProcessorService
     }
   end
 
-  def create_issue(params)
-    response = linear_client.create_issue(params)
+  def create_issue(params, user = nil)
+    response = linear_client.create_issue(params, user)
     return response if response[:error]
 
     {
@@ -33,8 +33,8 @@ class Integrations::Linear::ProcessorService
     }
   end
 
-  def link_issue(link, issue_id, title)
-    response = linear_client.link_issue(link, issue_id, title)
+  def link_issue(link, issue_id, title, user = nil)
+    response = linear_client.link_issue(link, issue_id, title, user)
     return response if response[:error]
 
     {