Fix fleet cattle healthcheck

[mantis-toboggan-md]

Summary

Fixes #12259

Occurred changes and/or fixed issues

This PR tweaks the logic used to show fleet and cattle agent statuses on the cluster explorer dashboard. Previously we checked that the user was able to list deployments; now we check that they can see the namespace the deployments would be in.

Technical notes summary

Areas or cases that should be tested

Cluster explorer dashboard with both local and downstream clusters (local will not show cattle agent), both as admin and a user with project membership only, as outlined in #12259

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[aalves08]

I gave this a test with both admin and standard users I local and rk2 digital ocean cluster and everything seems to fine. Codewise it also looks good, so overall LGTM!