Update Self-Hosted AIG authBypassExternal

A design partner reports the following:

The AI Gateway Helm chart has a comment stating that authBypassExternal is necessary for the self hosted AI Gateway, and it defaults to true. This behavior makes the ai-gateway API available to unauthenticated users. I would recommend removing that comment and setting the default to false as the self hosted AI gateway does work with auth enabled.

• with auth enabled, the Duo healthcheck returns the following error:
• System exchange A code snippet must be passable to the AI-gateway for users to utilize GitLab Duo in their IDE. Authentication with the AI gateway services failed: AI Gateway returned code 401: {"error":"Forbidden by auth provider"}
• I'm not sure what that is as Duo still seems to work with that error.