Track inherited paginated rollout prefixes (#33930)

## What changed

- Add `HistoryPosition` to identify a source thread and an exclusive rollout
  prefix by ordinal and JSONL byte offset.
- Add optional `history_base` metadata to `SessionMeta` so a thread can record
  an inherited prefix of another paginated rollout.
- Default missing `history_base` metadata to `None` and omit it when unset to
  preserve compatibility with existing rollout metadata.

## Testing

- Verify that legacy session metadata without `history_base` deserializes with
  no inherited history position.

GitOrigin-RevId: 3b2b07a2dfee793d3b37b0a60529f856f167dbc1

codex-rs/thread-store/src/in_memory.rs

@@ -443,6 +443,7 @@ impl InMemoryThreadStore {
             memory_mode: matches!(params.metadata.memory_mode, ThreadMemoryMode::Disabled)
                 .then_some("disabled".to_string()),
             history_mode: params.history_mode,
+            history_base: None,
             subagent_history_start_ordinal: params.subagent_history_start_ordinal,
             multi_agent_version: params.multi_agent_version,
             context_window: Some(SessionContextWindow::new(params.initial_window_id.clone())),