Fix race condition in CloseAsync that kills reconnect connections

[PiDiBi]

Fix race condition in CloseAsync that kills reconnect connections

Problem

When a connection drops (e.g. TCP reset), SshSession.CloseAsync runs on the ProcessMessages threadpool thread and does two things in sequence:

1. Sets IsConnected = false (inside a lock)
2. Calls Protocol?.Disconnect() (outside the lock)

Meanwhile, a reconnect thread (application or test) observes IsConnected == false and immediately starts ReconnectAsync → ConnectAsync, which replaces the Protocol property with a new SshProtocol wrapping the new TCP stream.

The race: If the old CloseAsync thread's Protocol?.Disconnect() executes after the reconnect thread has replaced Protocol, it disconnects the new protocol/stream, killing the reconnect connection mid-key-exchange.

This manifests as:

• SshConnectionException: Session closed while encrypting. Reason: ConnectionLost
• The server receives KeyExchangeDhInitMessage but the connection dies before the reply can be sent
• Flaky failures in InteropWithSshServerTSLib(..., reconnect: True) CI tests

Root Cause

Thread A (ProcessMessages → CloseAsync)     Thread B (app → ReconnectAsync)
─────────────────────────────────────────   ─────────────────────────────────
lock { IsConnected = false; }
                                            sees IsConnected == false
                                            ConnectAsync → Protocol = new SshProtocol(newStream)
Protocol?.Disconnect();  ← disconnects
the NEW stream!

Fix

Capture the Protocol reference inside the lock, atomically with IsConnected = false. Then disconnect only the captured (old) reference outside the lock:

// Before (buggy):
lock (this.disposeCancellationSource)
{
    if (IsClosed || !IsConnected) return;
    IsConnected = false;
}
// ... later:
Protocol?.Disconnect();           // ← reads current Protocol, may be replaced

// After (fixed):
SshProtocol? protocolToDisconnect;
lock (this.disposeCancellationSource)
{
    if (IsClosed || !IsConnected) return;
    IsConnected = false;
    protocolToDisconnect = Protocol;   // ← captured under lock
}
// ... later:
protocolToDisconnect?.Disconnect();    // ← always disconnects the old protocol

Changes

File	Change
src/cs/Ssh/SshSession.cs	Capture Protocol reference inside lock before disconnecting
test/cs/Ssh.Test/ReconnectRaceTests.cs	New tests for immediate reconnect after disconnect

Testing

• All 21 reconnect tests pass (19 existing + 2 new)
• New tests:
  • ReconnectImmediatelyAfterDisconnect_ShouldNotFail — disconnects client stream and immediately reconnects without waiting for server to detect disconnect
  • ReconnectMultipleTimes_ShouldAlwaysSucceed — stress test with 3 rapid disconnect/reconnect cycles, verifies session works after all reconnections