Fix race between SendMessage and server-initiated key re-exchange

[radekvermirovsky]

Fixes #1764

The race

Session.SendMessage checks _keyExchangeCompletedWaitHandle before acquiring _socketWriteLock. When the server initiates a key re-exchange in that window, the message loop thread resets the wait handle and sends the client's SSH_MSG_KEXINIT (under the same write lock). A data message that already passed the check then acquires the lock and goes out on the wire after our KEXINIT, violating RFC 4253 section 7.1 ("Once a party has sent a SSH_MSG_KEXINIT message ... it MUST NOT send any messages other than: Transport layer generic messages ...; Algorithm negotiation messages ...; Specific key exchange method messages").

Strict servers such as ProFTPD mod_sftp then fail the exchange or drop the connection, producing the two failure variants reported in #1764 (Message type 93 is not valid / connection drop with Key exchange failed).

The fix

SendMessage now re-checks the wait handle while holding the write lock and goes back to waiting when a re-exchange has started in the meantime. Re-checking under the lock is required: making the reset + KEXINIT send atomic is not sufficient, because a sender can already be blocked on the lock with a stale check result.

The packet is now also built entirely under the write lock, so a completing re-exchange can no longer swap _clientCipher/_clientMac/compression in the middle of building a packet.

The diff looks bigger than it is because the packet-building body moved unchanged into SendMessageWithinWriteLock — reviewing with whitespace changes hidden shows the actual change is the retry loop.

Why #1774 could not reproduce it

The repro attempt in #1774 ran against the OpenSSH test server. OpenSSH queues non-key-exchange output while a re-exchange is in progress and tolerates client data that slips in, so the race stays invisible there. ProFTPD mod_sftp does not tolerate it, so this PR adds an integration test running concurrent SFTP uploads against a ProFTPD container (alpine:3.24, ProFTPD 1.3.9c) configured to re-key every 1 MB (SFTPRekey required 3600 1), giving the race many trials per upload.

Evidence

Same environment (Docker Desktop/WSL2, net10.0), 4 concurrent uploads of 128 MB files, 3 attempts per run:

	Result
Without the fix	7 of 8 runs failed with SshConnectionException: Key exchange failed
With the fix	4 of 4 runs passed

[radekvermirovsky]

A note on CI coverage: the new ProFTPD rekey test currently runs on the Linux job only. On the Windows jobs it reports inconclusive, because Docker on the Windows runners is in Windows containers mode and cannot run the Linux ProFTPD image.

If you'd like it to run there too, I'm happy to extend the "Setup SSH Server" step of the Windows Integration Tests .NET job to also build and start the ProFTPD container with Podman in WSL2 on a second fixed port (mirroring how the OpenSSH server is provided), and to switch the test to the same Windows-CI detection InfrastructureFixture uses instead of skipping. Just let me know if that's wanted, or if the Linux coverage is sufficient.