feat(event_bus): add EventCatalog class with register/get/all methods

surfsense_backend/tests/unit/event_bus/conftest.py

@@ -0,0 +1,25 @@
+"""Shared fixtures for the ``app.event_bus`` unit-test tree.
+
+The event-type catalog is a module-level registry populated at import. Tests
+that register their own event types (or assert on registry contents) snapshot
+and restore it so state never leaks between tests.
+"""
+
+from __future__ import annotations
+
+from collections.abc import Iterator
+
+import pytest
+
+from app.event_bus.catalog import catalog
+
+
+@pytest.fixture
+def isolated_event_catalog() -> Iterator[None]:
+    """Snapshot and restore the event-type catalog around a test."""
+    snapshot = dict(catalog._registry)
+    try:
+        yield
+    finally:
+        catalog._registry.clear()
+        catalog._registry.update(snapshot)