feat(fusion_accounting_reports): drill_down_resolver service

Pure-Python helper that, given an account_id and a date range, fetches
posted account.move.line records and returns a flat list of dicts ready
for the drill-down OWL dialog. Used by the engine's drill_down() method.

3 new tests, 35 total passing.

Made-with: Cursor

fusion_accounting_reports/tests/__init__.py

@@ -2,3 +2,4 @@ from . import test_services_unit
 from . import test_currency_conversion
 from . import test_fusion_report
 from . import test_line_resolver
+from . import test_drill_down_resolver