test(sessions): add replay consistency harness for sessions memory and summaries

Add a multi-backend replay consistency test framework that drives
InMemory and SQLite backends with the same deterministic replay
cases, normalizes non-business differences, compares snapshots,
and writes a structured diff report.

Key features:
- 10 replay cases: single/multi-turn, tool calls, state updates,
  memory write/read, summary generation/overwrite/truncation,
  track events, concurrent writes, error recovery.
- Normalized snapshot comparison for events, state, memory,
  summaries, and tracks.
- Structured diff report (session_memory_summary_diff_report.json).
- Supports InMemory and SQLite backends.
- Mirrors the Go implementation in trpc-agent-go/session/replaytest/
  for shared fixtures and cross-language verification.

Resolves #89

Author: coder-mtj

tests/sessions/replay_consistency/__init__.py

@@ -0,0 +1,7 @@
+# Replay consistency test framework for Session / Memory / Summary.
+#
+# This package drives multiple backend implementations with the same
+# deterministic replay cases, normalizes non-business differences,
+# compares snapshots, and writes a structured diff report.
+#
+# Mirrors the Go implementation in trpc-agent-go/session/replaytest/.