[client] Deduplicate SSH PTY session setup and host key verification

Extract the identical PTY session setup shared by the wasm and Android
terminal clients into ssh.StartPTYSession, and move the stored-key host
verification onto the engine so the embed client delegates and the
Android client passes the engine directly as HostKeyVerifier.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

client/ssh/session.go

@@ -0,0 +1,73 @@
+package ssh
+
+import (
+	"fmt"
+	"io"
+
+	log "github.com/sirupsen/logrus"
+	"golang.org/x/crypto/ssh"
+)
+
+// defaultTerminalModes are the PTY modes used by the interactive terminal clients.
+var defaultTerminalModes = ssh.TerminalModes{
+	ssh.ECHO:          1,
+	ssh.TTY_OP_ISPEED: 14400,
+	ssh.TTY_OP_OSPEED: 14400,
+	ssh.VINTR:         3,
+	ssh.VQUIT:         28,
+	ssh.VERASE:        127,
+}
+
+// PTYSession is an interactive shell session with a PTY and its I/O pipes.
+type PTYSession struct {
+	Session *ssh.Session
+	Stdin   io.WriteCloser
+	Stdout  io.Reader
+	Stderr  io.Reader
+}
+
+// StartPTYSession opens a session on the client, requests an xterm-256color PTY
+// with the default terminal modes, wires up the I/O pipes and starts a shell.
+// The session is closed on any error.
+func StartPTYSession(client *ssh.Client, cols, rows int) (*PTYSession, error) {
+	session, err := client.NewSession()
+	if err != nil {
+		return nil, fmt.Errorf("new session: %w", err)
+	}
+
+	pty, err := setupPTYSession(session, cols, rows)
+	if err != nil {
+		if closeErr := session.Close(); closeErr != nil {
+			log.Debugf("ssh: session close after setup error: %v", closeErr)
+		}
+		return nil, err
+	}
+	return pty, nil
+}
+
+// setupPTYSession requests the PTY, opens the pipes and starts the shell on an
+// already created session.
+func setupPTYSession(session *ssh.Session, cols, rows int) (*PTYSession, error) {
+	if err := session.RequestPty("xterm-256color", rows, cols, defaultTerminalModes); err != nil {
+		return nil, fmt.Errorf("request pty: %w", err)
+	}
+
+	stdin, err := session.StdinPipe()
+	if err != nil {
+		return nil, fmt.Errorf("stdin pipe: %w", err)
+	}
+	stdout, err := session.StdoutPipe()
+	if err != nil {
+		return nil, fmt.Errorf("stdout pipe: %w", err)
+	}
+	stderr, err := session.StderrPipe()
+	if err != nil {
+		return nil, fmt.Errorf("stderr pipe: %w", err)
+	}
+
+	if err := session.Shell(); err != nil {
+		return nil, fmt.Errorf("start shell: %w", err)
+	}
+
+	return &PTYSession{Session: session, Stdin: stdin, Stdout: stdout, Stderr: stderr}, nil
+}