fix(session): make InMemorySessionStore.messageID atomic (#3592)

[aheritier]

Summary

Fixes #3592 — InMemorySessionStore.messageID was a plain int64 incremented
without synchronization while the store is accessed concurrently (HTTP handlers

• the runtime PersistenceObserver). Lost increments could produce duplicate
  message IDs, which in turn let UpdateMessage (which searches by ID) edit the
  wrong message.

What changed

• messageID is now atomic.Int64, incremented via Add(1). ID semantics are
  preserved: the counter starts at zero and Add(1) yields 1, 2, …, matching
  the previous pre-increment behavior.

Testing

• pkg/session/store_messageid_race_test.go — 200 concurrent AddMessage
  calls assert all returned IDs are unique. Verified to fail (duplicate IDs +
  data-race warnings, all iterations) against the pre-fix plain-int64 code and
  pass now.
• task dev + go test -race -count=2 ./pkg/session/...: green (validated in
  isolation).