Fix Duo Agent Platform probe's token generation and duplicate gRPC calls

What does this MR do and why?

Fix Duo Agent Platform probe's token generation and duplicate gRPC calls

The probe fetched its own Cloud Connector token via CloudConnector::Tokens.get, but Ai::DuoWorkflow::DuoWorkflowService::Client never read the token: kwarg always derived its own bearer token through Gitlab::AiGateway.headers. Drop the unused fetch and pass feature_setting through to the client instead, so the token/headers used for the real gRPC call are self-hosted aware.

Bonus: Also stop invoking list_tools twice when the probe fails.

References

Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/603347 as an pre-requisite MR. Also related to https://gitlab.com/gitlab-org/gitlab/-/work_items/616057+

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