🐛 Handle goproxy polling errors

[sahitya-chandra]

What this PR does / why we need it:

internal/goproxy.Client.GetVersions used wait.PollUntilContextTimeout for retrying goproxy requests, but discarded the poll error. This PR returns poll errors so caller cancellation and poll timeouts are not hidden by the retry loop, while preserving the existing retry error behavior for HTTP/read failures.

A regression test covers an already-canceled context and expects context.Canceled.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

/area clusterctl

[kubernetes-prow]

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