Switch custom MeasurementDB integration tests to Report.run() #123

tests/impulse_reporting/integration/custom_measurement_db_report_test.py (added in #109) still calls determine_report() and persist_results(), which are deprecated since #122. The tests emit FutureWarnings and will break once the wrappers are removed.

To do

• determine_report() (lines 100, 120): replace with report.run(persist_results=False).
• determine_report() + persist_results() (lines 128-129, 134-135): replace with report.run().
• test_incremental_detection_reads_through_custom_db: after a persisting run(), aggregation_dfs re-detects containers against the updated gold, so assert on the gold measurement_dimension table instead (container ids {1, 2}, container 1's _created_at unchanged).

Done when the file runs without deprecation warnings and all four tests pass.