feat(run-store,testcontainers): execution-snapshot read comparator and shared test utilities (#4772)

## Summary

Adds the read comparator for the in-progress migration of the run
execution-snapshot log from Postgres to Redis. The comparator samples a
single read against both stores, normalizes the two results to one
shape, and reports any per-field difference with a tagged metric. It
never serves a read itself: the diff layer imports only types, so it
cannot hold a store client, and a test enforces that by failing if any
value import appears.

Also adds a combined Postgres-and-Redis test fixture and two shared test
utilities (a cluster-slot assertion and a generic fault-injection
harness) that the parallel Redis-store work reuses.

Everything here is inert. Nothing constructs the comparator, so merging
changes no runtime behavior. It becomes active only when a later change
turns on compare mode.

## Notes

The divergence classes separate real differences (scalar, ordering,
waitpoint id set, validity, missing on one side) from two expected
classes that must not be driven to zero: a rotated idempotency key, and
a Redis-only surplus at a since-cursor tie. The since comparison is
direction sensitive: a Postgres-only entry at the cursor is always a
lost write, never an expected tie.