Fix: Make swagger doc compliant to OpenAPI (#3394)

This fixes issues in the swagger.json file. The motivation to do so is to be able to generate API clients using https://openapi-generator.tech Doing so will require further changes to the api spec, but this seems like a good first step since it is now "valid" according to editor.swagger.io and openapi-generator validate.

Fixes #2806

swagger/definitions/resource/conversation.yml

@@ -5,6 +5,8 @@ properties:
     description: ID of the conversation
   messages:
     type: array
+    items:
+      $ref: '#/definitions/message'
   account_id:
     type: number
     description: Account Id
@@ -21,7 +23,7 @@ properties:
   contact_last_seen_at:
     type: string
   agent_last_seen_at:
-    type: agent_last_seen_at
+    type: string
   unread_count:
     type: number
     description: The number of unread messages