Terminal: deliver large pastes whole, keep keystrokes in order, and keep one session's failure off the shared socket

- A message larger than the 64 KiB receive buffer was skipped and its tail dispatched as a frame of its
  own. Messages are now reassembled up to 16 MiB; one beyond that is dropped whole.
- Input frames are handed to the pump without waiting, and ShellStream takes no lock around its write
  buffer, so a snippet's text and its carriage return could interleave or lose bytes. Writes are
  serialised per pump in arrival order.
- A resize reaching a shell SSH.NET had already disposed threw out of the receive loop and closed the
  socket every tab shares. Each frame's failure now stays with its session.

tests/DodoSSH.Client.Terminal.Tests/FakeShellSession.cs

@@ -78,6 +78,14 @@ internal sealed class FakeShellSession : ISshShellSession
     /// <summary>The last size the pump forwarded, or null if it forwarded none.</summary>
     public TerminalSize? LastResize { get; private set; }
 
+    /// <summary>How long the first write takes, so a test can see whether a second one overtakes it.</summary>
+    public TimeSpan FirstWriteDelay { get; init; }
+
+    /// <summary>Whether a resize throws, as SSH.NET's does once it has disposed the shell's stream.</summary>
+    public bool ResizeThrows { get; init; }
+
+    private int writes;
+
     /// <summary>How many resizes were forwarded, so a dropped one is observable.</summary>
     public int ResizeCount { get; private set; }
 
@@ -119,19 +127,27 @@ internal sealed class FakeShellSession : ISshShellSession
     }
 
     /// <inheritdoc />
-    public ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken)
+    public async ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken)
     {
+        if (Interlocked.Increment(ref writes) == 1 && FirstWriteDelay > TimeSpan.Zero)
+        {
+            await Task.Delay(FirstWriteDelay, cancellationToken).ConfigureAwait(false);
+        }
+
         lock (gate)
         {
             written.AddRange(data.ToArray());
         }
-
-        return ValueTask.CompletedTask;
     }
 
     /// <inheritdoc />
     public void Resize(TerminalSize size)
     {
+        if (ResizeThrows)
+        {
+            throw new ObjectDisposedException("ShellStream");
+        }
+
         // Mirrors the real session: an unusable size is dropped rather than forwarded.
         if (!size.IsUsable)
         {