Prevent TLS1.2 handshake fragments from entering TLS1.3 post-handshak…

[FreezB11]

Description

Fixes #5624.

s2n_post_handshake_recv() treated any TLS_HANDSHAKE-typed record as a post-handshake message, even if the handshake hadn't actually finished. If a caller's completion tracking diverges from s2n-tls's internal state (e.g. the peer's final Finished message wasn't fully consumed yet), leftover handshake bytes get misread as a post-handshake message header — resulting in either S2N_ERR_BAD_MESSAGE, or worse, silent success if the misread bytes happened to look like a TLS_HELLO_REQUEST (which is silently ignored by default).

Fix

s2n_post_handshake_recv() now requires s2n_handshake_is_complete(conn) before processing anything, returning the existing S2N_ERR_HANDSHAKE_NOT_COMPLETE otherwise.

I deliberately checked handshake completeness rather than gating on actual_protocol_version < S2N_TLS13 (as suggested in the issue). TLS1.2 HelloRequest is a legitimate message this same function handles, so a blanket version check would make that unreachable. s2n_handshake_is_complete() correctly distinguishes leftover/unconsumed bytes (the bug) from a genuinely-finished TLS1.2 connection receiving HelloRequest or a finished TLS1.3 connection receiving KeyUpdate/NewSessionTicket (which already have their own version checks).

Testing

• New regression test (s2n_post_handshake_test.c) reproducing the exact scenario: negotiates a real TLS1.2 handshake, stalls the client right before it reads the server's Finished, then asserts the leftover bytes are rejected with S2N_ERR_HANDSHAKE_NOT_COMPLETE.
• Updated existing tests that built bare connections without a real handshake to use the existing s2n_skip_handshake() helper, matching real-world state.
• s2n_early_data_io_test.c already covered a variant of this bug class; it now correctly gets S2N_ERR_HANDSHAKE_NOT_COMPLETE instead of S2N_ERR_BAD_MESSAGE.
• One test asserted HelloRequest was accepted on a TLS1.3 connection — only passing because the handshake was never marked NEGOTIATED, bypassing a pre-existing version check. Updated to assert the correct rejection.
• 283/283 unit test binaries pass locally (ctest -j$(nproc)).

[FreezB11]

@kaukabrizvi @jmayclin, please do review when free thanks