fix(test): update run_slot_injection mock for new tuple return

PR 1.5 (commit 4168167) changed run_slot_injection's signature from
returning str to returning tuple[str, dict] so the scheduler can get
at the agentic message list for receipt persistence. The
test_run_slot_morning_runs_on_work_day mock was still returning the
old plain-string value, which unpacked to a ValueError at runtime.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_briefing_scheduler.py

@@ -84,7 +84,7 @@ async def test_run_slot_morning_runs_on_work_day():
          patch("fabledassistant.services.user_profile.get_profile", new_callable=AsyncMock, return_value=fake_profile), \
          patch("fabledassistant.services.settings.get_setting", new_callable=AsyncMock, return_value="UTC"), \
          patch("fabledassistant.services.briefing_conversations.get_or_create_today_conversation", new_callable=AsyncMock) as mock_conv, \
-         patch("fabledassistant.services.briefing_pipeline.run_slot_injection", new_callable=AsyncMock, return_value="") as mock_inject, \
+         patch("fabledassistant.services.briefing_pipeline.run_slot_injection", new_callable=AsyncMock, return_value=("", {})) as mock_inject, \
          patch("fabledassistant.services.briefing_conversations.post_message", new_callable=AsyncMock), \
          patch("fabledassistant.services.push.send_push_notification", new_callable=AsyncMock):