CVE-2026-40923: Tekton Pipelines: VolumeMount path restriction bypass via missing filepath.Clean in /tekton/ check

A validation bypass in the VolumeMount path restriction allows mounting volumes under restricted /tekton/ internal paths by using .. path traversal components. The restriction check uses strings.HasPrefix without filepath.Clean, so a path like /tekton/home/../results passes validation but resolves to /tekton/results at runtime.