feat: an HA pair's DHCP peers and config sync as facts (4.11.0)

get_dhcp_ha (declared on DhcpServerMixin, optional) is what a DHCP
server's own HA configuration says: on or off, this server's peer name,
each peer's name, role as the server words it, and HA URL.
ConfigSyncMixin.get_config_sync is where a host pushes its configuration
and for which services, by the names get_services reports; sync items
without a service name stay visible under `unmapped`. A driver mixes it
in, no role promises it. Both raise when they could not read.

What the facts mean for a pair -- hot standby, which side to write to --
is the caller's (NetOrk/netork#1125).

tests/test_dhcp_ha_config_sync.py

@@ -0,0 +1,70 @@
+# -*- coding: utf-8 -*-
+"""Two facts an HA pair of servers reports about itself.
+
+`get_dhcp_ha` is what a DHCP server's own HA configuration says: the peers it
+pairs with and each peer's role, as the server words it. `get_config_sync` is
+where a host pushes its configuration and for which of its services. Both are
+declarations; what they mean for the pair -- hot standby, who to write to --
+is the caller's to work out.
+"""
+
+from __future__ import annotations
+
+import inspect
+
+import napalm_device_types as package
+from napalm_device_types import ConfigSyncMixin, DhcpServerMixin, FirewallDriver
+
+
+class TestDeclaredOnly:
+    def test_get_dhcp_ha_is_absent_until_a_driver_implements_it(self):
+        assert not hasattr(DhcpServerMixin, "get_dhcp_ha")
+
+    def test_get_config_sync_is_absent_until_a_driver_implements_it(self):
+        assert not hasattr(ConfigSyncMixin, "get_config_sync")
+
+    def test_config_sync_mixin_owns_nothing_callable(self):
+        own = [
+            name
+            for name, value in vars(ConfigSyncMixin).items()
+            if not name.startswith("__") and inspect.isfunction(value)
+        ]
+        assert own == []
+
+    def test_the_firewall_role_does_not_carry_config_sync(self):
+        """A driver mixes the capability in; the role does not promise it."""
+        assert not issubclass(FirewallDriver, ConfigSyncMixin)
+
+    def test_a_driver_that_implements_them_is_found_by_hasattr(self):
+        class Pair(ConfigSyncMixin, DhcpServerMixin):
+            def get_dhcp_ha(self):
+                return {"enabled": False, "peers": []}
+
+            def get_config_sync(self):
+                return {"peer": None, "services": [], "unmapped": []}
+
+        assert callable(getattr(Pair(), "get_dhcp_ha", None))
+        assert callable(getattr(Pair(), "get_config_sync", None))
+
+
+class TestTypes:
+    def test_are_exported(self):
+        for name in ("ConfigSyncMixin", "ConfigSyncDict", "DhcpHaDict", "DhcpHaPeerDict"):
+            assert name in package.__all__ and hasattr(package, name), name
+
+    def test_a_peer_needs_only_its_name_and_role(self):
+        """A future server may not have a URL to report; Kea does."""
+        from napalm_device_types import DhcpHaPeerDict
+
+        assert DhcpHaPeerDict.__required_keys__ == frozenset({"name", "role"})
+
+    def test_ha_needs_only_whether_it_is_on_and_the_peers(self):
+        from napalm_device_types import DhcpHaDict
+
+        assert DhcpHaDict.__required_keys__ == frozenset({"enabled", "peers"})
+
+    def test_config_sync_reports_what_it_could_not_name(self):
+        """A sync item the driver has no service name for is still a fact."""
+        from napalm_device_types import ConfigSyncDict
+
+        assert ConfigSyncDict.__required_keys__ == frozenset({"peer", "services", "unmapped"})