Persist and prewarm agent tasks per thread (#17978)

## Summary
- persist registered agent tasks in the session state update stream so
the thread can reuse them
- prewarm task registration once identity registration succeeds, while
keeping startup failures best-effort
- isolate the session-side task lifecycle into a dedicated module so
AgentIdentityManager and RegisteredAgentTask do not leak across as many
core layers

## Testing
- cargo test -p codex-core startup_agent_task_prewarm
- cargo test -p codex-core
cached_agent_task_for_current_identity_clears_stale_task
- cargo test -p codex-core record_initial_history_

codex-rs/state/src/runtime/threads.rs

@@ -954,7 +954,8 @@ SELECT
 pub(super) fn extract_dynamic_tools(items: &[RolloutItem]) -> Option<Option<Vec<DynamicToolSpec>>> {
     items.iter().find_map(|item| match item {
         RolloutItem::SessionMeta(meta_line) => Some(meta_line.meta.dynamic_tools.clone()),
-        RolloutItem::ResponseItem(_)
+        RolloutItem::SessionState(_)
+        | RolloutItem::ResponseItem(_)
         | RolloutItem::Compacted(_)
         | RolloutItem::TurnContext(_)
         | RolloutItem::EventMsg(_) => None,
@@ -964,7 +965,8 @@ pub(super) fn extract_dynamic_tools(items: &[RolloutItem]) -> Option<Option<Vec<
 pub(super) fn extract_memory_mode(items: &[RolloutItem]) -> Option<String> {
     items.iter().rev().find_map(|item| match item {
         RolloutItem::SessionMeta(meta_line) => meta_line.meta.memory_mode.clone(),
-        RolloutItem::ResponseItem(_)
+        RolloutItem::SessionState(_)
+        | RolloutItem::ResponseItem(_)
         | RolloutItem::Compacted(_)
         | RolloutItem::TurnContext(_)
         | RolloutItem::EventMsg(_) => None,