feat: SystemIdentityMixin reads a host's SMBIOS identity (4.9.0)

What ties a server to its BMC: the BMC reports the server's serial
number and system UUID, the host reads the same values from its DMI
tables. Both drivers that read DMI (napalm-linux, napalm-proxmox) had a
copy of the shell for it, and neither read the board serial.

SystemIdentityMixin.get_system_identity() reads product_uuid,
product_serial and board_serial in one framed command and returns them
with vendor placeholders empty: a Supermicro board's product serial is
0123456789 on every one of them, others read "To be filled by O.E.M."
or "Default string", and many cheap boards share the UUID
03000200-0400-0500-0006-000700080009. is_placeholder_identity() is the
same filter for a BMC driver's side. A driver supplies only
_run_system_identity_command(command, privileged=); the files are
root's.

tests/test_system_identity.py

@@ -0,0 +1,102 @@
+"""SystemIdentityMixin: the SMBIOS identity of a host, for matching it to its BMC."""
+
+from __future__ import annotations
+
+import pytest
+
+from napalm_device_types import (
+    SYSTEM_IDENTITY_COMMAND,
+    SystemIdentityMixin,
+    is_placeholder_identity,
+    parse_system_identity,
+)
+
+UUID = "31303043-534D-C40C-7A72-6F1300000000"
+
+
+def _out(uuid: str, product_serial: str, board_serial: str) -> str:
+    return f"IDBEGIN\n{uuid}\n{product_serial}\n{board_serial}\nIDEND\n"
+
+
+# ── parsing ───────────────────────────────────────────────────────────────────
+
+
+def test_reads_uuid_and_both_serials():
+    assert parse_system_identity(_out(UUID, "S123", "ZM155S026022")) == {
+        "system_uuid": "31303043-534d-c40c-7a72-6f1300000000",
+        "product_serial": "S123",
+        "board_serial": "ZM155S026022",
+    }
+
+
+def test_a_supermicro_placeholder_serial_is_dropped():
+    # pve-dual, a Supermicro X10DRL-i: the product serial is the board vendor's default.
+    identity = parse_system_identity(_out(UUID, "0123456789", "ZM155S026022"))
+    assert identity["product_serial"] == ""
+    assert identity["board_serial"] == "ZM155S026022"
+
+
+@pytest.mark.parametrize(
+    "value",
+    ["", "None", "N/A", "Not Specified", "To be filled by O.E.M.", "Default string",
+     "System Serial Number", "0123456789", "1234567890", "0", "00000000", "Not Applicable"],
+)
+def test_placeholders(value):
+    assert is_placeholder_identity(value)
+
+
+@pytest.mark.parametrize(
+    "uuid",
+    ["00000000-0000-0000-0000-000000000000", "FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF",
+     # A UUID many cheap boards ship, the same on every one of them.
+     "03000200-0400-0500-0006-000700080009"],
+)
+def test_placeholder_uuids(uuid):
+    assert parse_system_identity(_out(uuid, "", ""))["system_uuid"] == ""
+
+
+def test_real_values_are_not_placeholders():
+    assert not is_placeholder_identity("ZM155S026022")
+    assert not is_placeholder_identity(UUID)
+
+
+def test_unreadable_files_read_as_empty():
+    # product_serial and product_uuid are root's alone; without root they come back empty.
+    assert parse_system_identity(_out("", "", "BS1")) == {
+        "system_uuid": "", "product_serial": "", "board_serial": "BS1"}
+
+
+def test_output_without_its_frame_is_not_a_reading():
+    with pytest.raises(ValueError):
+        parse_system_identity("sudo: a password is required\n")
+
+
+def test_terminal_noise_around_the_frame_is_ignored():
+    assert parse_system_identity("\x1b[?2004h" + _out(UUID, "S1", "B1"))["board_serial"] == "B1"
+
+
+# ── the mixin ─────────────────────────────────────────────────────────────────
+
+
+class _Host(SystemIdentityMixin):
+    def __init__(self, output: str):
+        self.output = output
+        self.calls: list = []
+
+    def _run_system_identity_command(self, command: str, *, privileged: bool) -> str:
+        self.calls.append((command, privileged))
+        return self.output
+
+
+def test_the_driver_only_carries_the_command_across():
+    host = _Host(_out(UUID, "S1", "B1"))
+
+    assert host.get_system_identity()["board_serial"] == "B1"
+    ((command, privileged),) = host.calls
+    assert command == SYSTEM_IDENTITY_COMMAND
+    assert privileged is True
+
+
+def test_the_command_reads_the_three_dmi_files():
+    for name in ("product_uuid", "product_serial", "board_serial"):
+        assert f"/sys/class/dmi/id/{name}" in SYSTEM_IDENTITY_COMMAND