Add team_assignment_limit_change activity type to OpenAPI spec

Adds OpenAPI documentation for team assignment limit changes in activity logs API.

Changes:
- Adds team_assignment_limit_change to activity_type enum
- Adds team (object with id and name) metadata field
- Adds old_assignment_limit (nullable integer) metadata field
- Adds new_assignment_limit (nullable integer) metadata field

Related PRs:
- intercom/intercom#445742 (API presenter implementation)
- intercom/intercom#445704 (Event infrastructure)

Related Issue: intercom/intercom#445916

Follows pattern from PR #293 (admin assignment limits).

Author: salmanbshah

descriptions/0/api.intercom.io.yaml

@@ -15109,6 +15109,7 @@ components:
           - app_team_creation
           - app_team_deletion
           - app_team_membership_modification
+          - team_assignment_limit_change
           - app_timezone_change
           - app_webhook_creation
           - app_webhook_deletion
@@ -15230,6 +15231,29 @@ components:
           nullable: true
           description: The ticket assignment limit value for an admin.
           example: 20
+        team:
+          type: object
+          nullable: true
+          description: Details about the team whose assignment limit was changed.
+          properties:
+            id:
+              type: integer
+              description: The ID of the team.
+              example: 123
+            name:
+              type: string
+              description: The name of the team.
+              example: Support Team
+        old_assignment_limit:
+          type: integer
+          nullable: true
+          description: The previous team assignment limit value (null if no limit was set).
+          example: null
+        new_assignment_limit:
+          type: integer
+          nullable: true
+          description: The new team assignment limit value (null if limit was removed).
+          example: 50
     addressable_list:
       title: Addressable List
       type: object