Preserve paginated history for spawned subagents (#33432)

## What changed

- Inherit paginated history mode when spawning or forking a subagent from a paginated parent.
- Load the parent's model context when forking and persist it as an inherited rollout prefix, while excluding that prefix from the child's projected turns, items, and metadata.
- Record the boundary between inherited context and child history, and reject partially initialized paginated subagent rollouts on resume.
- Resume paginated subagents from their stored model context instead of legacy rollout history.

## Testing

Added coverage for fresh and forked paginated subagents, compacted-history forks, model-context resume, inherited-prefix projection, metadata isolation, and incomplete-prefix detection.

GitOrigin-RevId: e57dc37c29aa4aaaf57b052b991be591d730e5ba

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(),
+                subagent_history_start_ordinal: None,
                 multi_agent_version: None,
                 context_window: None,
             },
@@ -2248,6 +2249,7 @@ mod tests {
                 selected_capability_roots: Vec::new(),
                 memory_mode: None,
                 history_mode: Default::default(),
+                subagent_history_start_ordinal: None,
                 multi_agent_version: None,
                 context_window: None,
             },