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/core/tests/suite/otel.rs

@@ -228,6 +228,7 @@ async fn process_sse_records_failed_event_when_stream_closes_without_completed()
                 .features
                 .disable(Feature::GhostCommit)
                 .expect("test config should allow feature update");
+            config.model_provider.stream_max_retries = Some(0);
         })
         .build(&server)
         .await
@@ -406,6 +407,7 @@ async fn process_sse_failed_event_logs_missing_error() {
                 .features
                 .disable(Feature::GhostCommit)
                 .expect("test config should allow feature update");
+            config.model_provider.stream_max_retries = Some(0);
         })
         .build(&server)
         .await