curl: CVE-2026-7168: cross-proxy Digest auth state leak

Summary: On libcurl 8.19.0, Proxy Digest state learned from proxyA survives an independent transfer boundary on a reused easy handle and is emitted preemptively to proxyB when the proxy is changed. In the attached C PoC, the first CONNECT to proxyB carries Proxy-Authorization: Digest ... built fr...