fix: add missing slash after port in registry proxy URL replacement

[mayooot]

What issue type does this pull request address? (keep at least one, remove the others)
/kind bugfix

What does this pull request do? Which issues does it resolve? (use resolves #<issue_number> if possible)
resolves #3379

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Note

Ensures correct URL rewriting in the registry reverse proxy to avoid malformed paths and redirects.

• Refactors inline replaceHost into newHostRewriter to consistently rewrite target to http://<newHost>/
• Updates replacement to include a trailing slash after the port (e.g., http://127.0.0.1:15000/)
• Adds proxy_test.go with TestNewHostRewriterAddsSlashAfterPort to verify the rewrite behavior
• Applies the new rewriter in proxy.go via rewriteHeaderTransport.replaceHost

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