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/protocol/src/protocol.rs

@@ -3049,6 +3049,16 @@ impl SessionContextWindow {
     }
 }
 
+/// Exclusive position in another thread's paginated rollout history.
+#[derive(Serialize, Deserialize, Clone, Copy, Debug, PartialEq, Eq, JsonSchema, TS)]
+pub struct HistoryPosition {
+    pub thread_id: ThreadId,
+    /// First rollout ordinal not included from the prefix file.
+    pub end_ordinal_exclusive: u64,
+    /// Byte offset immediately after the last included JSONL record from the prefix file.
+    pub end_byte_offset: u64,
+}
+
 /// SessionMeta contains session-level data that doesn't correspond to a specific turn.
 ///
 /// NOTE: There used to be an `instructions` field here, which stored user_instructions, but we
@@ -3098,6 +3108,9 @@ pub struct SessionMeta {
     pub memory_mode: Option<String>,
     #[serde(default)]
     pub history_mode: ThreadHistoryMode,
+    /// Exclusive prefix of another paginated rollout inherited by this thread.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub history_base: Option<HistoryPosition>,
     /// First rollout ordinal that belongs to this subagent's own projected history.
     ///
     /// Earlier rollout records are inherited model context and stay out of child
@@ -3134,6 +3147,7 @@ impl Default for SessionMeta {
             selected_capability_roots: Vec::new(),
             memory_mode: None,
             history_mode: ThreadHistoryMode::default(),
+            history_base: None,
             subagent_history_start_ordinal: None,
             multi_agent_version: None,
             context_window: None,
@@ -5854,6 +5868,7 @@ mod tests {
         }))?;
 
         assert_eq!(session_meta.history_mode, ThreadHistoryMode::Legacy);
+        assert_eq!(session_meta.history_base, None);
         let serialized = serde_json::to_value(&session_meta)?;
         assert_eq!(serialized["history_mode"], json!("legacy"));
         let mut unknown = serialized;