quic: expose handshake confirmation state

[dillona]

Summary

Expose QUIC handshake confirmation through the public Rust and C APIs.

This adds:

• Connection::is_handshake_confirmed() to the Rust API;
• quiche_conn_is_handshake_confirmed() to the C API; and
• documentation describing the difference between handshake completion and confirmation.

For servers, the handshake is confirmed immediately after it completes. For clients, it is confirmed after receiving HANDSHAKE_DONE.

Motivation

Connection::is_established() reports that the TLS handshake is complete, but applications may need to know when the QUIC handshake is confirmed. In particular, clients may need to wait for confirmation before performing operations such as connection migration.

The confirmation state already exists internally, but callers currently cannot inspect it. This addresses the API request in #1489 and supports use cases related to the migration requirements discussed in #2631.

This PR only exposes the existing state; it does not change migration enforcement or handshake behavior.

Testing

• Updated the existing handshake-confirmation test to use the public Rust accessor.
• Added an FFI test verifying that the C and Rust APIs report the same state before and after the handshake.