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/state/src/runtime/threads.rs

@@ -2183,6 +2183,7 @@ mod tests {
                 selected_capability_roots: Vec::new(),
                 memory_mode: Some("polluted".to_string()),
                 history_mode: Default::default(),
+                history_base: None,
                 subagent_history_start_ordinal: None,
                 multi_agent_version: None,
                 context_window: None,
@@ -2249,6 +2250,7 @@ mod tests {
                 selected_capability_roots: Vec::new(),
                 memory_mode: None,
                 history_mode: Default::default(),
+                history_base: None,
                 subagent_history_start_ordinal: None,
                 multi_agent_version: None,
                 context_window: None,