[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/events.rs

@@ -803,7 +803,6 @@ pub(crate) struct CodexTurnEventParams {
     pub(crate) status: Option<TurnStatus>,
     pub(crate) turn_error: Option<CodexErrorInfo>,
     pub(crate) codex_error_kind: Option<CodexErrKind>,
-    pub(crate) codex_error_subreason: Option<String>,
     pub(crate) codex_error_http_status_code: Option<u16>,
     pub(crate) steer_count: Option<usize>,
     pub(crate) total_tool_call_count: Option<usize>,