Fix SSH command shells dying on cmd_exec with trailing newline

[rudraditya21]

Fixes: #20642

Summary

• Treat a trailing newline as an explicit command terminator in SingleCommandShell token execution.
• Prevents SSH command shells from injecting a separator after a newline, which could terminate non‑PTY sessions.
• Adds a small spec for the newline case.

Local Test Setup

1. Build a minimal SSH server:

   rm -rf /tmp/ssh-nosh
   mkdir -p /tmp/ssh-nosh

   ssh-keygen -t ed25519 -f /tmp/ssh-nosh/id_ed25519 -N ""
   cat /tmp/ssh-nosh/id_ed25519.pub > /tmp/ssh-nosh/authorized_keys

   cat <<'EOF' > /tmp/ssh-nosh/sshd_config
   Port 2222
   Protocol 2
   PubkeyAuthentication yes
   AuthorizedKeysFile .ssh/authorized_keys
   PermitRootLogin no
   PasswordAuthentication no
   KbdInteractiveAuthentication no
   ChallengeResponseAuthentication no
   UsePAM no
   AllowUsers git
   LogLevel VERBOSE
   Subsystem sftp /usr/lib/openssh/sftp-server
   EOF

   cat <<'EOF' > /tmp/ssh-nosh/Dockerfile
   FROM ubuntu:22.04
   RUN apt-get update && apt-get install -y openssh-server && mkdir /var/run/sshd
   RUN useradd -m -s /bin/bash git && passwd -d git && usermod -U git
   RUN mkdir -p /home/git/.ssh
   COPY authorized_keys /home/git/.ssh/authorized_keys
   COPY sshd_config /etc/ssh/sshd_config
   RUN chmod 700 /home/git/.ssh && chmod 600 /home/git/.ssh/authorized_keys && chown -R git:git /home/git/.ssh
   EXPOSE 2222
   CMD ["/usr/sbin/sshd","-D","-e"]
   EOF

   docker build -t ssh-nosh /tmp/ssh-nosh
   docker run --rm -p 2222:2222 ssh-nosh

2. In msfconsole:

   use auxiliary/scanner/ssh/ssh_login
   set RHOSTS 127.0.0.1
   set RPORT 2222
   set USERNAME git
   set PRIVATE_KEY file:/tmp/ssh-nosh/id_ed25519
   set CreateSession true
   run
   sessions -i <id>
   irb
   self.shell_command_token("id\n")

Expected behavior

• The command runs and the SSH session remains alive.
• Prior to this change, the trailing newline could terminate the session.