fix(agent/agentssh): use deterministic host key for SSH server (#16626)

Fixes: https://github.com/coder/coder/issues/16490

The Agent's SSH server now initially generates fixed host keys and, once it receives its manifest, generates and replaces that host key with the one derived from the workspace ID, ensuring consistency across agent restarts. This prevents SSH warnings and host key verification errors when connecting to workspaces through Coder Desktop.

While deterministic keys might seem insecure, the underlying Wireguard tunnel already provides encryption and anti-spoofing protection at the network layer, making this approach acceptable for our use case.

---
Change-Id: I8c7e3070324e5d558374fd6891eea9d48660e1e9
Signed-off-by: Thomas Kosiewski <tk@coder.com>

cli/ssh_test.go

@@ -453,6 +453,71 @@ func TestSSH(t *testing.T) {
 		<-cmdDone
 	})
 
+	t.Run("DeterministicHostKey", func(t *testing.T) {
+		t.Parallel()
+		client, workspace, agentToken := setupWorkspaceForAgent(t)
+		_, _ = tGoContext(t, func(ctx context.Context) {
+			// Run this async so the SSH command has to wait for
+			// the build and agent to connect!
+			_ = agenttest.New(t, client.URL, agentToken)
+			<-ctx.Done()
+		})
+
+		clientOutput, clientInput := io.Pipe()
+		serverOutput, serverInput := io.Pipe()
+		defer func() {
+			for _, c := range []io.Closer{clientOutput, clientInput, serverOutput, serverInput} {
+				_ = c.Close()
+			}
+		}()
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		inv, root := clitest.New(t, "ssh", "--stdio", workspace.Name)
+		clitest.SetupConfig(t, client, root)
+		inv.Stdin = clientOutput
+		inv.Stdout = serverInput
+		inv.Stderr = io.Discard
+
+		cmdDone := tGo(t, func() {
+			err := inv.WithContext(ctx).Run()
+			assert.NoError(t, err)
+		})
+
+		keySeed, err := agent.WorkspaceKeySeed(workspace.ID, "dev")
+		assert.NoError(t, err)
+
+		signer, err := agentssh.CoderSigner(keySeed)
+		assert.NoError(t, err)
+
+		conn, channels, requests, err := ssh.NewClientConn(&stdioConn{
+			Reader: serverOutput,
+			Writer: clientInput,
+		}, "", &ssh.ClientConfig{
+			HostKeyCallback: ssh.FixedHostKey(signer.PublicKey()),
+		})
+		require.NoError(t, err)
+		defer conn.Close()
+
+		sshClient := ssh.NewClient(conn, channels, requests)
+		session, err := sshClient.NewSession()
+		require.NoError(t, err)
+		defer session.Close()
+
+		command := "sh -c exit"
+		if runtime.GOOS == "windows" {
+			command = "cmd.exe /c exit"
+		}
+		err = session.Run(command)
+		require.NoError(t, err)
+		err = sshClient.Close()
+		require.NoError(t, err)
+		_ = clientOutput.Close()
+
+		<-cmdDone
+	})
+
 	t.Run("NetworkInfo", func(t *testing.T) {
 		t.Parallel()
 		client, workspace, agentToken := setupWorkspaceForAgent(t)