fix(session): make InMemorySessionStore.messageID increment atomic

[aheritier]

Summary

pkg/session/store.go:137 is a plain int64 incremented at :284 with no atomic/mutex, while the store is hit concurrently (HTTP handlers + runtime PersistenceObserver). Lost increments → duplicate message IDs → UpdateMessage (searches by ID, store.go:290-314) can edit the wrong message.

Fix

Use atomic.Int64 (or a mutex) for the counter.

Acceptance

• Concurrent AddMessage test asserts unique IDs; -race clean.