dapr mcpservers: skip cert-expiry check in self-hosted mode, validate --output

[Mukuwul]

Description

Two fixes for the new dapr mcpservers command:

• Self-hosted runs no longer touch the cluster. PostRun called kubernetes.CheckForCertExpiry() unconditionally, so a purely local resources-directory listing reached for kubeconfig/cluster access. It's now guarded behind kubernetesMode, matching what dapr list does.
• --output is validated. dapr mcpservers -o table used to be silently accepted and behave like the default; it now fails in PreRun with the valid values (json, yaml, list), matching dapr list's behavior.

The -n shorthand clash described in the issue (name here vs namespace in init/uninstall/workflow/scheduler) is deliberately not touched - changing a shorthand is breaking, so that part needs a maintainer call first.

Issue reference

Fixes #1675

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests (no e2e harness exercises mcpservers yet; happy to add coverage if wanted)
• Extended the documentation (n/a - behavior fix)

[Mukuwul]

The red check here is darwin_amd64_complete failing on TestWorkflowChildCalls/fan_out_workflow - the test-scheduler app times out waiting for actor in-flight lock claims during rebalancing. That's unrelated to this change (which only adds --output validation and guards the PostRun cert-expiry check behind kubernetesMode); it's the same flaky workflow test that's been going red intermittently on the macOS runner. A re-run should clear it.