Remove util.URLJoin and replace all callers with direct path concatenation (#36867)

`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.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <2114189+wxiaoguang@users.noreply.github.com>

modules/util/url.go

@@ -5,7 +5,6 @@ package util
 
 import (
 	"net/url"
-	"path"
 	"strings"
 )
 
@@ -19,29 +18,6 @@ func PathEscapeSegments(path string) string {
 	return escapedPath
 }
 
-// URLJoin joins url components, like path.Join, but preserving contents
-// Deprecated: it has unclear behaviors, should not be used anymore. It is only used in some tests.
-// Need to be removed in the future.
-func URLJoin(base string, elems ...string) string {
-	if !strings.HasSuffix(base, "/") {
-		base += "/"
-	}
-	baseURL, err := url.Parse(base)
-	if err != nil {
-		return ""
-	}
-	joinedPath := path.Join(elems...)
-	argURL, err := url.Parse(joinedPath)
-	if err != nil {
-		return ""
-	}
-	joinedURL := baseURL.ResolveReference(argURL).String()
-	if !baseURL.IsAbs() && !strings.HasPrefix(base, "/") {
-		return joinedURL[1:] // Removing leading '/' if needed
-	}
-	return joinedURL
-}
-
 func SanitizeURL(s string) (string, error) {
 	u, err := url.Parse(s)
 	if err != nil {