`util.URLJoin` was deprecated with unclear semantics (path normalization
via `url.Parse`/`ResolveReference` that surprised callers). This removes
it entirely and replaces all usages with straightforward `"/"` string
concatenation.
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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>