[Security Solution][Serverless] Add to case present in assistant chat for cases none privilege

[MadameSheema]

Originally reported by @jamesspi

Describe the bug:

• Add to case present in assistant chat for users without cases privileges

Precondition:

• To have a user without cases privileges

*Steps to reproduce:**

1. Open the AI assistant with the customised user

Current behavior:

• In an assistant chat, the “add to case” button is still available to click on, but the user is then presented with the following:

Expected behavior:

• The add to case button should not be displayed

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)