kcp front-proxy does not strip inbound X-Remote-* identity headers, allowing any authenticated client to inject groups/warrants and impersonate system:masters in any workspace

Summary The kcp front-proxy fails to strip client-supplied identity headers before forwarding requests to shards. Any authenticated tenant can inject their own X-Remote-Group and X-Remote-Extra- headers, which the shard trusts as a verified identity assertion — allowing a low-privilege user to...