Make calendar outbox invariants explicit

src/govoplan_calendar/backend/outbox.py

@@ -980,7 +980,9 @@ def execute_calendar_outbox_operation(
         )
         return operation
     except CalDAVPreconditionFailed as exc:
-        assert client is not None
+        if client is None:
+            _fail_operation(session, operation=operation, source=source, error=exc)
+            return operation
         try:
             matched, remote_etag = _remote_matches_operation(client, operation)
         except Exception:
@@ -1142,7 +1144,8 @@ def retry_calendar_outbox_operation(
     unavailable_reason = _operation_source_unavailable_reason(operation, source)
     if unavailable_reason:
         raise ValueError(unavailable_reason)
-    assert source is not None
+    if source is None:
+        raise RuntimeError("Calendar outbox source availability invariant was violated")
     operation.status = "pending"
     operation.attempt_count = 0
     operation.available_at = utcnow()
@@ -1186,7 +1189,8 @@ def reconcile_calendar_outbox_operation(
     unavailable_reason = _operation_source_unavailable_reason(operation, source)
     if unavailable_reason:
         raise ValueError(unavailable_reason)
-    assert source is not None
+    if source is None:
+        raise RuntimeError("Calendar outbox source availability invariant was violated")
     if client_factory is None:
         from govoplan_calendar.backend.service import caldav_client_for_source
 
@@ -1248,7 +1252,8 @@ def discard_calendar_outbox_operation(
         source is None or source.tenant_id != operation.tenant_id
     ):
         raise ValueError(unavailable_reason)
-    assert source is not None
+    if source is None:
+        raise RuntimeError("Calendar outbox source availability invariant was violated")
     now = utcnow()
     unresolved_chain = (
         session.query(CalendarOutboxOperation)