Load a config entry offline from the last discovery snapshot (#295)

An appliance switched off at the wall used to take its whole config entry
down with it: async_setup_entry raised ConfigEntryNotReady, so the device
read as failed and its entities existed only as registry rows until the
appliance came back.

Loading the entry anyway isn't enough on its own. Entities here are the
output of discovery, discovery only runs inside a successful poll, and
platforms enumerate `bound` exactly once at forward time -- so an entry
that loads while offline loads empty, and with no listeners subscribed the
base coordinator stops rescheduling and never polls again.

Bank the resources dict each successful first cycle hands _run_discovery,
along with the subdevice candidate list and the /oic identity that route
the registry, and replay it through _run_discovery when the first refresh
fails. Storing the poll input rather than a rendered entity list keeps one
implementation of discovery instead of two: the offline entity set is
produced by the same code that produced the live one.

Three things fall out of that:

- Platforms judge entity existence against `discovery_resources`, not the
  live cache. The live cache deliberately stays empty, which is what keeps
  a restored entity `unavailable` rather than rendering a stale value for
  an appliance nobody can currently reach.
- A live discovery that disagrees with the snapshot reloads the entry --
  platforms can't adopt a changed set in place, so a firmware update or a
  sibling subdevice that starts answering needs a fresh setup.
- The entry holds one coordinator listener for its lifetime, so polling is
  scheduled regardless of how many entities are live.

An entry that has never reached the device has no snapshot, keeps raising
ConfigEntryNotReady, and closes its session on the way out as before -- no
metadata to build a device from, and it leaves room for setup flows that
need to interact with the appliance (#168).

Restores the two tests PR #303 rewrote, narrowed to that no-snapshot path.

tests/test_entity.py

@@ -29,6 +29,15 @@ class _FakeCoordinator:
         # subdevices (issue #177).
         return self.last_resources
 
+    # _is_included judges existence against the discovery view, which is the
+    # live cache for everything but an offline load (issue #295).
+    @property
+    def discovery_resources(self):
+        return self.last_resources
+
+    def discovery_canonical(self, subdevice):
+        return self.canonical_resources(subdevice)
+
 
 def _coord(last_resources) -> LocalThingsCoordinator:
     return cast(LocalThingsCoordinator, _FakeCoordinator(last_resources))