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/protocol/src/protocol.rs

@@ -2765,6 +2765,26 @@ impl fmt::Display for SubAgentSource {
     }
 }
 
+/// Persisted agent-task details that let a resumed thread keep using the same backend task.
+///
+/// `agent_runtime_id` is validation metadata for the globally registered agent identity, not a
+/// separate session-scoped identity. Resume only restores this task after confirming that runtime
+/// id still matches the globally registered identity; otherwise the cached task is discarded and a
+/// fresh task can be registered.
+#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq, JsonSchema, TS)]
+pub struct SessionAgentTask {
+    pub agent_runtime_id: String,
+    pub task_id: String,
+    pub registered_at: String,
+}
+
+/// Session-scoped state updates that can be appended after the canonical SessionMeta line.
+#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq, JsonSchema, TS, Default)]
+pub struct SessionStateUpdate {
+    #[serde(default)]
+    pub agent_task: Option<SessionAgentTask>,
+}
+
 /// 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
@@ -2834,6 +2854,7 @@ pub struct SessionMetaLine {
 #[serde(tag = "type", content = "payload", rename_all = "snake_case")]
 pub enum RolloutItem {
     SessionMeta(SessionMetaLine),
+    SessionState(SessionStateUpdate),
     ResponseItem(ResponseItem),
     Compacted(CompactedItem),
     TurnContext(TurnContextItem),