[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/core/src/codex_delegate.rs

@@ -109,6 +109,7 @@ pub(crate) async fn run_codex_thread_interactive(
     emit_subagent_session_started(
         &parent_session.services.analytics_events_client,
         client_metadata,
+        codex.session.session_id(),
         codex.session.conversation_id,
         Some(parent_session.conversation_id),
         thread_config,