feat(stovepipe): persist request history entries - #648

Summary:
Intent:
- Make the request history schema usable through a backend-neutral, queue-scoped persistence contract.
- This change builds on #645, which introduces the append-only MySQL table.

Changes:
- Define validated state and event history entries with bounded outcome reasons.
- Add create, point-read, and chronological cursor-list storage operations.
- Implement and wire the MySQL store, generated mocks, and reusable backend contract tests.

Test Plan:
- Exercised create, duplicate detection, point reads, queue isolation, and equal-timestamp cursor pagination against MySQL 8.

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