Fix TGUI46 scheduler ground truth

[panDing19]

Problem

TGUI46 asks the agent to identify and disable db_migration_user_preferences as JOB-712, but the scheduler fixture currently maps JOB-712 to weekly_cache_warmup. The migration job is present under a duplicated JOB-705, so a correct agent that inspects the scheduler fixture can reach the opposite conclusion from the grader/reference.

Affected tasks

• benchmark/gui/TGUI46_job_failure_triage_workmail_sms_notes

Fix

• Move db_migration_user_preferences onto JOB-712 with the 02:30 overlap details referenced by the task.
• Move the unrelated weekly cache warmup row to JOB-727 so JOB-712 resolves to the expected root-cause job.
• Add a regression test that pins the TGUI46 scheduler fixture ground truth.

Tests

• uv run pytest tests/test_service_schema_drift.py::test_tgui46_scheduler_fixture_matches_reference_ground_truth -q