[codex] Fix preparation start boundary

[jjoonleo]

Summary

• handle schedules received exactly at preparationStartTime through the official ScheduleStarted path
• defensively start schedules when a timer target is already at or before now
• add boundary tests for exact receive and early-start sessions

Fixes #362

Validation

• flutter test test/presentation/app/bloc/schedule/schedule_bloc_test.dart
• dart analyze lib/presentation/app/bloc/schedule/schedule_bloc.dart test/presentation/app/bloc/schedule/schedule_bloc_test.dart
• git diff --check

Note: full flutter analyze still reports existing unrelated widgetbook/generated-file errors.

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[github-actions]

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