CVE-2026-59221: open-webui terminal proxy path traversal guard bypass via 9x encoded traversal

The fix for GHSA-r2wg-2mcr-66rv is incomplete in v0.9.6 and current main. backend/open_webui/routers/terminals.py documents _sanitize_proxy_path() as decoding until stable, but the implementation stops after 8 unquote() passes. A 9x percent-encoded ../... path parameter remains once-encoded after the loop, passes the posixpath.normpath() and cleaned.startswith('..') checks, and is forwarded to the configured terminal server. The upstream server then receives a decoded traversal path such as /base/../admin/system.