[codex-analytics] add tool review event schema (#18747)

## Why

We want to emit terminal review analytics for tool-related approval
flows, but the event contract needs to exist before the reducer can
publish anything.

This PR is the schema-only slice for the Codex review event family.

## What changed

- add the `ReviewEvent` analytics envelope in
`codex-rs/analytics/src/events.rs`
- define the review subject kind, reviewer, trigger, terminal status,
and post-review resolution enums
- define the review event payload with thread, turn, item, lineage,
tool, and timing fields that the emitter stack will populate

## Verification

- stacked verification in dependent PRs: `cargo test -p codex-analytics
analytics_client_tests --manifest-path codex-rs/Cargo.toml`

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/18747).
* #18748
* #21434
* __->__ #18747
* #17090
* #17089
* #20514

codex-rs/analytics/src/analytics_client_tests.rs

@@ -1012,7 +1012,7 @@ fn command_execution_event_serializes_expected_shape() {
                     runtime_os_version: "15.3.1".to_string(),
                     runtime_arch: "aarch64".to_string(),
                 },
-                thread_source: Some("user"),
+                thread_source: Some(ThreadSource::User),
                 subagent_source: None,
                 parent_thread_id: None,
                 tool_name: "shell".to_string(),