bug: incorrect pagination in Memory Session Manager `list_messages`

[Hweinstock]

Problem

Before PR #244

State events and conversational events were naturally separated by actorId:

# create_session used a prefixed actorId
create_event(actorId="session_my-session-id", payload=[{"blob": ...}])

# create_agent used a prefixed actorId
create_event(actorId="agent_my-agent-id", payload=[{"blob": ...}])

# create_message used the regular actorId
create_event(actorId="my-actor-id", payload=[{"conversational": ...}])

So when list_messages called list_events with actorId="my-actor-id", it only got conversational events back. State events lived under different actorIds and were invisible to this query. max_results counted only
what mattered.

After PR #244

Everything now uses the same actorId, with metadata distinguishing state events:

# create_session — same actorId, metadata marks it as state
create_event(
    actorId="my-actor-id",  # ← same as messages now
    metadata={"stateType": {"stringValue": "SESSION"}},
    payload=[{"blob": ...}],
)


# create_agent — same actorId, metadata marks it as state
create_event(
    actorId="my-actor-id",  # ← same as messages now
    metadata={"stateType": {"stringValue": "AGENT"}, "agentId": {"stringValue": "..."}},
    payload=[{"blob": ...}],
)

# create_message — same actorId, no metadata
create_event(actorId="my-actor-id", payload=[{"conversational": ...}])

Potential Issues

list_messages in session_manager.py fetches events without any metadata filter:

events = self.memory_client.list_events(
    memory_id=self.config.memory_id,
    actor_id=self.config.actor_id,
    session_id=session_id,
    max_results=max_results,  # ← this now includes state events in the count
)

Since all events share the same actorId now, this query returns both conversational AND state events. The API returned max_results events total, some of which were state events. Those get discarded by the converter, so the final list is shorter than what the caller asked for.

Concrete example

A session with 1 session state event, 2 agent state events, and 20 conversational messages. Customer calls list_messages(limit=10):

1. max_results = limit + offset = 10
2. list_events returns 10 events: 3 state + 7 conversational
3. events_to_messages discards the 3 state events → 7 messages
4. Customer gets 7 messages, expected 10

Customer does not get a pagination token, so they assume 7 is all that exists, and doesn't know there are more.

Acceptance Criteria

• customer should be given nextToken, or should always get maxResults exactly if more results exist.
• unit tests and/or integration verifying fix of the bug.