[codex-analytics] add grouped session id to runtime events (#24655)

## Why
- Runtime analytics events report `thread_id`, which identifies the
individual thread emitting an event
- They don't report `session_id`, which identifies the shared session
for a root thread and its subagent threads
- Emitting both identifiers allows analytics to group related activity

## What Changed
- Adds `session_id` to relevant analytics events (thread_initalized,
turn, turn_steer, compaction, guardian_review)
- Tracks each thread's session ID in the analytics reducer so subsequent
thread scoped events emit the same value
- Carries the shared session ID through subagent initialization

## Verification
- `just test -p codex-analytics` validates event payloads and subagent
session grouping.
- Focused `codex-app-server` tests validate session IDs for thread,
turn, and steer events.
- Focused `codex-core` tests validate root and subagent session ID
propagation.

codex-rs/app-server/tests/suite/v2/analytics.rs

@@ -168,11 +168,13 @@ pub(crate) fn thread_initialized_event(payload: &Value) -> Result<&Value> {
 pub(crate) fn assert_basic_thread_initialized_event(
     event: &Value,
     thread_id: &str,
+    session_id: &str,
     expected_model: &str,
     initialization_mode: &str,
     expected_thread_source: &str,
 ) {
     assert_eq!(event["event_params"]["thread_id"], thread_id);
+    assert_eq!(event["event_params"]["session_id"], session_id);
     assert_eq!(
         event["event_params"]["app_server_client"]["product_client_id"],
         DEFAULT_CLIENT_NAME

codex-rs/app-server/tests/suite/v2/thread_fork.rs

@@ -405,7 +405,14 @@ async fn thread_fork_tracks_thread_initialized_analytics() -> Result<()> {
 
     let payload = wait_for_analytics_payload(&server, DEFAULT_READ_TIMEOUT).await?;
     let event = thread_initialized_event(&payload)?;
-    assert_basic_thread_initialized_event(event, &thread.id, "mock-model", "forked", "user");
+    assert_basic_thread_initialized_event(
+        event,
+        &thread.id,
+        &thread.session_id,
+        "mock-model",
+        "forked",
+        "user",
+    );
     Ok(())
 }
 

codex-rs/app-server/tests/suite/v2/thread_resume.rs

@@ -422,7 +422,14 @@ async fn thread_resume_tracks_thread_initialized_analytics() -> Result<()> {
 
     let payload = wait_for_analytics_payload(&server, DEFAULT_READ_TIMEOUT).await?;
     let event = thread_initialized_event(&payload)?;
-    assert_basic_thread_initialized_event(event, &thread.id, "gpt-5.3-codex", "resumed", "user");
+    assert_basic_thread_initialized_event(
+        event,
+        &thread.id,
+        &thread.session_id,
+        "gpt-5.3-codex",
+        "resumed",
+        "user",
+    );
     assert_eq!(event["event_params"]["thread_source"], "user");
     Ok(())
 }

codex-rs/app-server/tests/suite/v2/thread_start.rs

@@ -439,7 +439,14 @@ async fn thread_start_tracks_thread_initialized_analytics() -> Result<()> {
     let payload = wait_for_analytics_payload(&server, DEFAULT_READ_TIMEOUT).await?;
     assert_eq!(payload["events"].as_array().expect("events array").len(), 1);
     let event = thread_initialized_event(&payload)?;
-    assert_basic_thread_initialized_event(event, &thread.id, "mock-model", "new", "user");
+    assert_basic_thread_initialized_event(
+        event,
+        &thread.id,
+        &thread.session_id,
+        "mock-model",
+        "new",
+        "user",
+    );
     Ok(())
 }
 

codex-rs/app-server/tests/suite/v2/turn_start.rs

@@ -862,6 +862,7 @@ async fn turn_start_tracks_turn_event_analytics() -> Result<()> {
 
     let event = wait_for_analytics_event(&server, DEFAULT_READ_TIMEOUT, "codex_turn_event").await?;
     assert_eq!(event["event_params"]["thread_id"], thread.id);
+    assert_eq!(event["event_params"]["session_id"], thread.session_id);
     assert_eq!(event["event_params"]["turn_id"], turn.id);
     assert_eq!(
         event["event_params"]["app_server_client"]["product_client_id"],

codex-rs/app-server/tests/suite/v2/turn_steer.rs

@@ -306,6 +306,7 @@ async fn turn_steer_returns_active_turn_id() -> Result<()> {
     let event =
         wait_for_analytics_event(&server, DEFAULT_READ_TIMEOUT, "codex_turn_steer_event").await?;
     assert_eq!(event["event_params"]["thread_id"], thread.id);
+    assert_eq!(event["event_params"]["session_id"], thread.session_id);
     assert_eq!(event["event_params"]["result"], "accepted");
     assert_eq!(event["event_params"]["num_input_images"], 0);
     assert_eq!(event["event_params"]["expected_turn_id"], turn.id);