test: add pytestmark and full coverage for events bus (#917)

- Add `pytestmark = pytest.mark.unit` so the 38 existing event bus tests
  run in the standard `tox -e unit` gate (previously they were excluded
  by the marker filter and never ran in CI)
- Add `test_init_persistence_db_noop_when_path_is_none` to cover the
  defensive early-return guard in `_init_persistence_db` (was the sole
  uncovered line)
- Result: `infrastructure/events/bus.py` at 100% coverage; unit suite
  grows from 474 → 513 tests

Fixes #917

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/infrastructure/test_event_bus.py

@@ -7,6 +7,8 @@ from unittest.mock import patch
 import pytest
 
 import infrastructure.events.bus as bus_module
+
+pytestmark = pytest.mark.unit
 from infrastructure.events.bus import (
     Event,
     EventBus,
@@ -352,6 +354,14 @@ class TestEventBusPersistence:
         events = bus.replay()
         assert events == []
 
+    def test_init_persistence_db_noop_when_path_is_none(self):
+        """_init_persistence_db() is a no-op when _persistence_db_path is None."""
+        bus = EventBus()
+        # _persistence_db_path is None by default; calling _init_persistence_db
+        # should silently return without touching the filesystem.
+        bus._init_persistence_db()  # must not raise
+        assert bus._persistence_db_path is None
+
     async def test_wal_mode_on_persistence_db(self, persistent_bus):
         """Persistence database should use WAL mode."""
         conn = sqlite3.connect(str(persistent_bus._persistence_db_path))