Currently, CMO only tests that the plugin Deployment is rolled out with the appropriate config https://github.com/openshift/cluster-monitoring-operator/blob/f7e92e869c43fa0455d656dcfc89045b60e5baa1/test/e2e/config_test.go#L730

The plugin Deployment does set any readinessProbe, we're missing a check to ensure the plugin is ready to serve requests.

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With the new plugin backend, a readiness probe can/will be added, see https://github.com/openshift/cluster-monitoring-operator/pull/2412#issuecomment-2315085438, that will help ensure minimal readiness on palyload tests flavors.

The CMO test can be more demanding and ask for /plugin-manifest.json