Handle the unspecified dispatch event type explicitly

mypy's `exhaustive-match` check (enabled by the `frequenz-repo-config`
0.19.0 migration) reports that the `match` over the event type in the
dispatch stream is missing a case for `Event.UNSPECIFIED`, the enum's
zero value.

The dispatch API should never actually send this, but the enum allows
for it, and the `match` previously covered it only by falling through
silently. Add an explicit case, so a malformed event logs a warning and
gets skipped instead of crashing the whole background service, and the
lack of behaviour beyond that is a documented decision rather than an
oversight.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

src/frequenz/dispatch/_bg_service.py

@@ -341,6 +341,16 @@ async def _run(self) -> None:  # pylint: disable=too-many-branches
                                     await self._lifecycle_events_tx.send(
                                         Deleted(dispatch=new_dispatch)
                                     )
+                                case Event.UNSPECIFIED:
+                                    # The API should never send this in
+                                    # practice, but the enum allows for it,
+                                    # so log and ignore it rather than crash
+                                    # the whole service on garbage input.
+                                    _logger.warning(
+                                        "Received dispatch event with an "
+                                        "unspecified event type: %s",
+                                        selected.message,
+                                    )
 
                         case StreamRetrying():
                             is_retry_attempt = True