[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/analytics/src/events.rs

@@ -147,6 +147,7 @@ pub(crate) struct CodexRuntimeMetadata {
 #[derive(Serialize)]
 pub(crate) struct ThreadInitializedEventParams {
     pub(crate) thread_id: String,
+    pub(crate) session_id: String,
     pub(crate) app_server_client: CodexAppServerClientMetadata,
     pub(crate) runtime: CodexRuntimeMetadata,
     pub(crate) model: String,
@@ -420,6 +421,7 @@ impl GuardianReviewAnalyticsResult {
 
 #[derive(Serialize)]
 pub(crate) struct GuardianReviewEventPayload {
+    pub(crate) session_id: String,
     pub(crate) app_server_client: CodexAppServerClientMetadata,
     pub(crate) runtime: CodexRuntimeMetadata,
     #[serde(flatten)]
@@ -738,6 +740,7 @@ pub(crate) struct CodexHookRunEventRequest {
 #[derive(Serialize)]
 pub(crate) struct CodexCompactionEventParams {
     pub(crate) thread_id: String,
+    pub(crate) session_id: String,
     pub(crate) turn_id: String,
     pub(crate) app_server_client: CodexAppServerClientMetadata,
     pub(crate) runtime: CodexRuntimeMetadata,
@@ -767,6 +770,7 @@ pub(crate) struct CodexCompactionEventRequest {
 #[derive(Serialize)]
 pub(crate) struct CodexTurnEventParams {
     pub(crate) thread_id: String,
+    pub(crate) session_id: String,
     pub(crate) turn_id: String,
     // TODO(rhan-oai): Populate once queued/default submission type is plumbed from
     // the turn/start callsites instead of always being reported as None.
@@ -821,6 +825,7 @@ pub(crate) struct CodexTurnEventRequest {
 #[derive(Serialize)]
 pub(crate) struct CodexTurnSteerEventParams {
     pub(crate) thread_id: String,
+    pub(crate) session_id: String,
     pub(crate) expected_turn_id: Option<String>,
     pub(crate) accepted_turn_id: Option<String>,
     pub(crate) app_server_client: CodexAppServerClientMetadata,
@@ -926,6 +931,7 @@ pub(crate) fn codex_plugin_metadata(plugin: PluginTelemetryMetadata) -> CodexPlu
 
 pub(crate) fn codex_compaction_event_params(
     input: CodexCompactionEvent,
+    session_id: String,
     app_server_client: CodexAppServerClientMetadata,
     runtime: CodexRuntimeMetadata,
     thread_source: Option<ThreadSource>,
@@ -934,6 +940,7 @@ pub(crate) fn codex_compaction_event_params(
 ) -> CodexCompactionEventParams {
     CodexCompactionEventParams {
         thread_id: input.thread_id,
+        session_id,
         turn_id: input.turn_id,
         app_server_client,
         runtime,
@@ -1026,6 +1033,7 @@ pub(crate) fn subagent_thread_started_event_request(
 ) -> ThreadInitializedEvent {
     let event_params = ThreadInitializedEventParams {
         thread_id: input.thread_id,
+        session_id: input.session_id,
         app_server_client: CodexAppServerClientMetadata {
             product_client_id: input.product_client_id,
             client_name: Some(input.client_name),