chore: Fix agent API returning 500 error (#9193)

- We were using the @user object in the view, which wasn't available when using api_access_token.
- Also replaced the object to @agent and added additional specs to confirm the right object

fixes: https://linear.app/chatwoot/issue/CW-3185/agent-create-api-throwing-500-response
ref: https://discord.com/channels/647412545203994635/1219928138163949568/1219928138163949568

app/controllers/api/v1/accounts/agents_controller.rb

@@ -19,7 +19,7 @@ class Api::V1::Accounts::AgentsController < Api::V1::Accounts::BaseController
       account: Current.account
     )
 
-    builder.perform
+    @agent = builder.perform
   end
 
   def update