add(core): safety check downgrade warning (#11964)

Add per-turn notice when a request is downgraded to a fallback model due
to cyber safety checks.

**Changes**

- codex-api: Emit a ServerModel event based on the openai-model response
header and/or response payload (SSE + WebSocket), including when the
model changes mid-stream.
- core: When the server-reported model differs from the requested model,
emit a single per-turn warning explaining the reroute to gpt-5.2 and
directing users to Trusted
    Access verification and the cyber safety explainer.
- app-server (v2): Surface these cyber model-routing warnings as
synthetic userMessage items with text prefixed by Warning: (and document
this behavior).

codex-rs/otel/src/traces/otel_manager.rs

@@ -748,6 +748,7 @@ impl OtelManager {
             ResponseEvent::ReasoningSummaryPartAdded { .. } => {
                 "reasoning_summary_part_added".into()
             }
+            ResponseEvent::ServerModel(_) => "server_model".into(),
             ResponseEvent::ServerReasoningIncluded(_) => "server_reasoning_included".into(),
             ResponseEvent::RateLimits(_) => "rate_limits".into(),
             ResponseEvent::ModelsEtag(_) => "models_etag".into(),