refactor: prepare the interactive shell in its own module

The prompt-hook step (#41) pushed linux.py past its line count in the
quality baseline. The three steps that make netmiko's fresh shell fit to
read - history off, prompt hooks off, no line wrap - and the prompt
pattern move to napalm_linux/_shell.py, under strict mypy. linux.py
shrinks below where it was; the baseline follows.

napalm_linux/linux.py

@@ -74,6 +74,9 @@ from netmiko.exceptions import (
     NetmikoTimeoutException,
 )
 
+from napalm_linux._shell import PROMPT_RE as _PROMPT_RE
+from napalm_linux._shell import prepare_shell
+
 logger = logging.getLogger("napalm_linux")
 
 # Package managers in detection order
@@ -86,9 +89,6 @@ _RC_MARKER = "__NETORK_RC="
 _RC_MARKER_RE = re.compile(rf"^{_RC_MARKER}(\d+)\s*$", re.MULTILINE)
 
 
-#: The end of a command's output when nothing better is known: a line that looks
-#: like a shell prompt.
-_PROMPT_RE = r"[#$\>]\s*$"
 #: A command ending in ``echo __NAME=$?`` reports its exit status on a line of its
 #: own: ``_RC_MARKER`` here, ``__APT_RC=`` and ``__SVC_RC=`` in napalm-device-types.
 _STATUS_ECHO_RE = re.compile(r"echo\s+(__[A-Z_]+=)\$\?")
@@ -159,18 +159,6 @@ _PRIVILEGED_STDIN = (
     "mktemp, cat) and have root install that file."
 )
 
-#: Sent before anything else in the interactive session (NetOrk/netork#1039):
-#: the shell writes no history file and records nothing more. The leading space
-#: keeps this line itself out of a history under ``HISTCONTROL=ignorespace``;
-#: a shell without ``set -o history`` (dash) only complains, to /dev/null.
-_HISTORY_OFF = " unset HISTFILE; set +o history 2>/dev/null; true"
-
-#: Sent right after ``_HISTORY_OFF`` (#41): systemd 258's
-#: ``/etc/profile.d/80-systemd-osc-context.sh`` wraps every command in OSC 3008
-#: sequences from ``PS0`` and ``PROMPT_COMMAND``, each with a fresh UUID. It appends
-#: to ``PROMPT_COMMAND`` as an array, so only ``unset`` removes its hook.
-_PROMPT_HOOKS_OFF = " unset PS0 PROMPT_COMMAND; true"
-
 #: First in every script ``_sudo`` runs: a sudo that does not prompt (NOPASSWD,
 #: a cached timestamp) leaves the password on stdin, and nothing may read it.
 _NO_STDIN = "exec </dev/null; "
@@ -419,32 +407,7 @@ class LinuxDriver(
             raise ConnectionException(str(exc)) from exc
         self._host_key.observe(self._device.remote_conn_pre.get_transport())
 
-        # No history before anything else is typed (NetOrk/netork#1039).
-        try:
-            self._device.send_command(_HISTORY_OFF, expect_string=_PROMPT_RE)
-        except Exception:
-            logger.debug(
-                "Could not switch the shell history off on %s", self.hostname, exc_info=True
-            )
-
-        # No systemd OSC 3008 context around every command (#41). netmiko learned
-        # the prompt with those sequences in front, and a prompt carrying a fresh
-        # UUID never matches again, so it reads the prompt anew once they are gone.
-        try:
-            self._device.send_command(_PROMPT_HOOKS_OFF, expect_string=_PROMPT_RE)
-            self._device.set_base_prompt()
-        except Exception:
-            logger.debug(
-                "Could not switch the prompt hooks off on %s", self.hostname, exc_info=True
-            )
-
-        # Prevent PTY from wrapping long output lines (e.g. docker JSON).
-        try:
-            self._device.send_command(
-                "stty cols 10000 2>/dev/null || true", expect_string=r"[#$>]\s*$"
-            )
-        except Exception:
-            logger.debug("%s: ignored", self.hostname, exc_info=True)
+        prepare_shell(self._device, self.hostname)
 
         self._pkg_manager = self._forced_pkg_manager or self._detect_pkg_manager()
         logger.debug("Connected to %s, pkg_manager=%s", self.hostname, self._pkg_manager)