Add test coverage for ClusterProvider.Get and its unavailable path

[magic-peach]

This repo has no PR template, so this is a plain factual description.

What changed: added tests to pkg/reconciler/common/multicluster_test.go covering ClusterProvider.Get (cache miss, a stale cached entry, a live cached entry) and doRefresh's generic ClusterProfile Get API error path.

Why: docs/multicluster.md documents ReasonClusterProfileUnavailable and ReasonRemoteClusterStale as status conditions under TargetClusterResolved=False, but neither had any test coverage anywhere in the package.

How tested: go build ./... and go test on pkg/reconciler/common, all green, including the new tests and the existing suite. Rebuilt and retested against the current tip of main immediately before opening this PR.

Confirmed no existing issue or PR covers this.

[knative-prow]

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[knative-prow]

Hi @magic-peach. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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