## What changed
Expose `connect_windows_proxy` for callers handling a CONNECT `407` response. Use libcurl with the signed-in Windows user's Negotiate or NTLM credentials, then return the authenticated connection with buffered data preserved.
Support HTTP and HTTPS proxies, with default or caller-provided TLS configuration. HTTPS proxies requiring authentication tokens bound to their TLS connection are unsupported.
Enable libcurl's NTLM support in Cargo and Bazel builds.
## Testing
Add a Windows-only test asserting that libcurl provides NTLM, SPNEGO, and SSPI support.
GitOrigin-RevId: 4ed7d2208766404b6125ce2f4b5f91de5616a37f
