Resolve "Health check for self-hosted Duo Agent Platform errors out"

What does this MR do and why?

Making the generate_token call required the DUO_WORKFLOW_SELF_SIGNED_JWT__SIGNING_KEY to be present in the AIGW image, which self-hosted customers will not. list_tools is better because it works without DUO_WORKFLOW_SELF_SIGNED_JWT__SIGNING_KEY being present.]

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Related to #571453 (closed)