[Bug]: Inactive session clears historical latest turn from thread shell #4399

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Start a thread turn so projection_threads.latest_turn_id and the session active_turn_id both reference the running turn.
2. Complete the turn and project thread.session-set with status ready and activeTurnId: null. Ensure no later thread.turn-diff-completed event is projected, or let the terminal session event arrive after the diff event.
3. Read the thread through ProjectionSnapshotQuery.getThreadShellById.

The persistence path is deterministic:

• applyThreadTurnsProjection correctly settles the running turn.
• applyThreadsProjection then writes event.payload.session.activeTurnId into projection_threads.latest_turn_id.
• Terminal session states carry activeTurnId: null, so the historical latest-turn pointer is erased.
• getLatestTurnRowByThread joins through that pointer and consequently returns no latest turn.

This became much easier to trigger after commit 501ce27, which correctly clears stale session activeTurnId for generic inactive session.state.changed events. The projection currently conflates the active session turn with the historical latest thread turn.

Expected behavior

An inactive session should have activeTurnId: null, while projection_threads.latest_turn_id should continue referencing the completed/interrupted/error latest turn. Shell snapshots should retain latestTurn and its completedAt.

Actual behavior

projection_threads.latest_turn_id becomes null. Shell snapshots lose latestTurn, so clients lose completion/unread state. The behavior appears intermittent because a later thread.turn-diff-completed event writes the turn ID back; whichever event is projected last wins.

A minimal fix appears to be preserving the existing pointer when the session has no active turn:

latestTurnId:
  event.payload.session.activeTurnId ?? existingRow.value.latestTurnId

This keeps session activity and historical thread state separate. A regression test should project running -> ready with no later checkpoint event and assert that getThreadShellById(...).latestTurn remains completed.

Impact

Minor bug or occasional failure

Version or commit

main @ 9d9208c; regression trigger 501ce27

Environment

macOS desktop; provider-independent orchestration projection path

Logs or stack traces

No exception is emitted; persisted projection state is internally inconsistent.

Workaround

A later thread.turn-diff-completed event may restore the pointer, but this is timing-dependent and unavailable for turns without a later checkpoint update.