Include session IDs in tool item analytics events (#32903)

## What changed

- Add `session_id` to the shared tool item event payload, sourced from the
  thread metadata.
- Preserve the parent session ID on tool events emitted by subagent threads.

## Testing

- Cover serialization of the session ID for command execution events,
  subagent tool events, and every supported completed tool item type.

GitOrigin-RevId: 73afc0e1be1ddec4fb5a42f43a23f1e87360e68f

codex-rs/analytics/src/events.rs

@@ -518,6 +518,7 @@ pub(crate) enum ToolItemFailureKind {
 #[derive(Serialize)]
 pub(crate) struct CodexToolItemEventBase {
     pub(crate) thread_id: String,
+    pub(crate) session_id: String,
     pub(crate) turn_id: String,
     /// App-server ThreadItem.id. For tool-originated items this generally
     /// corresponds to the originating core call_id.