test: improve event bus unit test coverage to 99% (Refs #1191)

Add tests for previously uncovered code paths in infrastructure.events.bus:
- TestSingletonHelpers: get_event_bus() singleton, init_event_bus_persistence()
  (idempotency and default-path behaviour), and module __getattr__ error path
- TestEventBusPersistence: exception-swallowing in _persist_event() and replay()
  (lines 143-144 and 200-202) via patched contextmanager

Coverage: 92.7% → 99.3% (38 tests, all passing).

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

tests/infrastructure/test_event_bus.py

@@ -1,10 +1,21 @@
 """Tests for the async event bus (infrastructure.events.bus)."""
 
 import sqlite3
+from pathlib import Path
+from unittest.mock import patch
 
 import pytest
 
-from infrastructure.events.bus import Event, EventBus, emit, event_bus, on
+import infrastructure.events.bus as bus_module
+from infrastructure.events.bus import (
+    Event,
+    EventBus,
+    emit,
+    event_bus,
+    get_event_bus,
+    init_event_bus_persistence,
+    on,
+)
 
 
 class TestEvent:
@@ -349,3 +360,111 @@ class TestEventBusPersistence:
             assert mode == "wal"
         finally:
             conn.close()
+
+    async def test_persist_event_exception_is_swallowed(self, tmp_path):
+        """_persist_event must not propagate SQLite errors."""
+        from unittest.mock import MagicMock
+
+        bus = EventBus()
+        bus.enable_persistence(tmp_path / "events.db")
+
+        # Make the INSERT raise an OperationalError
+        mock_conn = MagicMock()
+        mock_conn.execute.side_effect = sqlite3.OperationalError("simulated failure")
+
+        from contextlib import contextmanager
+
+        @contextmanager
+        def fake_ctx():
+            yield mock_conn
+
+        with patch.object(bus, "_get_persistence_conn", fake_ctx):
+            # Should not raise
+            bus._persist_event(Event(type="x", source="s"))
+
+    async def test_replay_exception_returns_empty(self, tmp_path):
+        """replay() must return [] when SQLite query fails."""
+        from unittest.mock import MagicMock
+
+        bus = EventBus()
+        bus.enable_persistence(tmp_path / "events.db")
+
+        mock_conn = MagicMock()
+        mock_conn.execute.side_effect = sqlite3.OperationalError("simulated failure")
+
+        from contextlib import contextmanager
+
+        @contextmanager
+        def fake_ctx():
+            yield mock_conn
+
+        with patch.object(bus, "_get_persistence_conn", fake_ctx):
+            result = bus.replay()
+            assert result == []
+
+
+# ── Singleton helpers ─────────────────────────────────────────────────────────
+
+
+class TestSingletonHelpers:
+    """Test get_event_bus(), init_event_bus_persistence(), and module __getattr__."""
+
+    def test_get_event_bus_returns_same_instance(self):
+        """get_event_bus() is a true singleton."""
+        a = get_event_bus()
+        b = get_event_bus()
+        assert a is b
+
+    def test_module_event_bus_attr_is_singleton(self):
+        """Accessing bus_module.event_bus via __getattr__ returns the singleton."""
+        assert bus_module.event_bus is get_event_bus()
+
+    def test_module_getattr_unknown_raises(self):
+        """Accessing an unknown module attribute raises AttributeError."""
+        with pytest.raises(AttributeError):
+            _ = bus_module.no_such_attr  # type: ignore[attr-defined]
+
+    def test_init_event_bus_persistence_sets_path(self, tmp_path):
+        """init_event_bus_persistence() enables persistence on the singleton."""
+        bus = get_event_bus()
+        original_path = bus._persistence_db_path
+        try:
+            bus._persistence_db_path = None  # reset for the test
+            db_path = tmp_path / "test_init.db"
+            init_event_bus_persistence(db_path)
+            assert bus._persistence_db_path == db_path
+        finally:
+            bus._persistence_db_path = original_path
+
+    def test_init_event_bus_persistence_is_idempotent(self, tmp_path):
+        """Calling init_event_bus_persistence() twice keeps the first path."""
+        bus = get_event_bus()
+        original_path = bus._persistence_db_path
+        try:
+            bus._persistence_db_path = None
+            first_path = tmp_path / "first.db"
+            second_path = tmp_path / "second.db"
+            init_event_bus_persistence(first_path)
+            init_event_bus_persistence(second_path)  # should be ignored
+            assert bus._persistence_db_path == first_path
+        finally:
+            bus._persistence_db_path = original_path
+
+    def test_init_event_bus_persistence_default_path(self):
+        """init_event_bus_persistence() uses 'data/events.db' when no path given."""
+        bus = get_event_bus()
+        original_path = bus._persistence_db_path
+        try:
+            bus._persistence_db_path = None
+            # Patch enable_persistence to capture what path it receives
+            captured = {}
+
+            def fake_enable(path: Path) -> None:
+                captured["path"] = path
+
+            with patch.object(bus, "enable_persistence", side_effect=fake_enable):
+                init_event_bus_persistence()
+
+            assert captured["path"] == Path("data/events.db")
+        finally:
+            bus._persistence_db_path = original_path