Reinitialize server MAC HashAlgorithm between packets to prevent spurious MAC errors

[Copilot]

Since 2026.0.0, Session.ReceiveMessage verifies the server MAC by reusing a single _serverMac HashAlgorithm instance across the whole session via TransformBlock/TransformFinalBlock, without ever calling Initialize() between packets. This violates HashAlgorithm's reuse contract. On certain older .NET Framework mscorlib.dll builds (pre-HMAC-rework, CAPI-backed implementations), the inner hash state isn't reset after TransformFinalBlock, so the second and subsequent MAC-protected packets fail — silently producing a wrong hash for hmac-sha2-* (SshConnectionException: MAC error) or throwing CryptographicException for hmac-sha1.

Fix

• In both the ETM and non-ETM MAC verification branches of Session.ReceiveMessage, call _serverMac.Initialize() right after computing/comparing the MAC and before evaluating whether to throw, so the instance is always ready for the next packet.
• Applied unconditionally, not gated to .NET Framework, since this addresses an actual HashAlgorithm reuse contract violation rather than a platform-specific quirk.

_ = _serverMac.TransformFinalBlock(Array.Empty<byte>(), 0, 0);

var macIsValid = CryptoAbstraction.FixedTimeEquals(_serverMac.Hash, ...);

// Not all HashAlgorithm implementations reset their internal state after
// TransformFinalBlock(), so we need to explicitly reinitialize the algorithm
// before it can be reused to compute the MAC of the next packet.
_serverMac.Initialize();

if (!macIsValid)
{
    throw new SshConnectionException("MAC error", DisconnectReason.MacError);
}

The client-side send path (_clientMac.TryComputeHash) already performs a one-shot hash computation per packet and is unaffected by this issue.

• Fixes MAC error / CryptographicException on .NET Framework in 2026.0.0: _serverMac is reused across packets without Initialize() #1829