sessions: add backend replay consistency test harness

Add standardized replay cases for session events, state updates,
memory operations, summary compression, and failure recovery across
InMemory, SQLite, Mock Redis, and optional real Redis backends.

Normalize non-business fields and generate field-level difference
reports while keeping summary ownership, version, and replacement
metadata strictly comparable.

Fixes #89

RELEASE NOTES: NONE

Author: Enjoykkk