Refuse a pinned host that answers with a key of another type, rather than asking whether to trust it

Pins are stored per algorithm, so an endpoint pinned for ssh-ed25519 that was answered with only an
ECDSA or RSA key found no pin and raised SshHostKeyUnknownException: the first-use question, where a
pin exists to produce a hard refusal. Anybody in the middle needed nothing more than a key of a
different type.

The connection factory now reads every pin for the endpoint before the handshake, offers the pinned
algorithms first so a server that still holds its pinned key negotiates it, and in the gate accepts a
key under an unpinned algorithm only when its fingerprint matches one of the endpoint's pins (one RSA
key signs as rsa-sha2-256 and rsa-sha2-512). Anything else is a mismatch. IKnownHostStore gains
FindForEndpointAsync for this.

src/DodoSSH.Client.Ssh/HostKeyTrust.cs

@@ -64,6 +64,22 @@ public interface IKnownHostStore
     /// </remarks>
     ValueTask<string?> FindAsync(string host, int port, string algorithm, CancellationToken cancellationToken);
 
+    /// <summary>Returns every key pinned for one endpoint, whatever the algorithm.</summary>
+    /// <remarks>
+    /// <para>
+    /// ◆ <b>What stops a pin being sidestepped by changing algorithm.</b> <see cref="FindAsync"/> answers
+    /// per algorithm, so on its own a host pinned for <c>ssh-ed25519</c> that is answered by somebody offering
+    /// only <c>ecdsa-sha2-nistp256</c> looks like a host never seen before — and "never seen before" is a
+    /// question the user is asked, where "changed" is a refusal. Knowing the endpoint has pins at all is what
+    /// lets the connection factory tell the two apart; see <c>SshNetConnectionFactory</c>.
+    /// </para>
+    /// <para>The same rule as <see cref="FindAsync"/>: no I/O.</para>
+    /// </remarks>
+    ValueTask<IReadOnlyList<HostKeyPresentation>> FindForEndpointAsync(
+        string host,
+        int port,
+        CancellationToken cancellationToken);
+
     /// <summary>Records a host key as trusted.</summary>
     ValueTask TrustAsync(HostKeyPresentation presentation, CancellationToken cancellationToken);
 
@@ -156,6 +172,22 @@ public sealed class InMemoryKnownHostStore : IKnownHostStore
         }
     }
 
+    /// <inheritdoc />
+    public ValueTask<IReadOnlyList<HostKeyPresentation>> FindForEndpointAsync(
+        string host,
+        int port,
+        CancellationToken cancellationToken)
+    {
+        lock (gate)
+        {
+            IReadOnlyList<HostKeyPresentation> found = pins.Values
+                .Where(pin => KnownHostIdentity.Comparer.Equals(pin.Host, host) && pin.Port == port)
+                .ToArray();
+
+            return ValueTask.FromResult(found);
+        }
+    }
+
     /// <inheritdoc />
     public ValueTask TrustAsync(HostKeyPresentation presentation, CancellationToken cancellationToken)
     {