[codex-analytics] stop sending codex error subreason (#27060)

## Summary
- stop emitting `codex_error_subreason` on `codex_turn_event`
- remove the transient analytics fact plumbing that copied
`CodexErr::InvalidRequest(String)` into the event
- update analytics serialization coverage accordingly

## Why
`codex_error_subreason` is a free-form copy of `InvalidRequest(String)`,
including raw provider 400 bodies in some paths. That makes it unsafe as
an analytics field because it can carry user-derived or sensitive text.

## Validation
- `just fmt`
- `just test -p codex-analytics`

codex-rs/analytics/src/analytics_client_tests.rs

@@ -3318,7 +3318,6 @@ fn turn_event_serializes_expected_shape() {
             status: Some(TurnStatus::Completed),
             turn_error: None,
             codex_error_kind: None,
-            codex_error_subreason: None,
             codex_error_http_status_code: None,
             steer_count: Some(0),
             total_tool_call_count: None,
@@ -3391,7 +3390,6 @@ fn turn_event_serializes_expected_shape() {
                 "status": "completed",
                 "turn_error": null,
                 "codex_error_kind": null,
-                "codex_error_subreason": null,
                 "codex_error_http_status_code": null,
                 "steer_count": 0,
                 "total_tool_call_count": null,
@@ -4098,10 +4096,6 @@ async fn turn_lifecycle_emits_failed_turn_event() {
         payload["event_params"]["codex_error_kind"],
         json!("invalid_request")
     );
-    assert_eq!(
-        payload["event_params"]["codex_error_subreason"],
-        json!("unknown turn environment id `env-2`")
-    );
     assert_eq!(
         payload["event_params"]["codex_error_http_status_code"],
         json!(null)