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/app-server/tests/suite/v2/mod.rs

@@ -15,6 +15,7 @@ mod plan_item;
 mod rate_limits;
 mod request_user_input;
 mod review;
+mod safety_check_downgrade;
 mod skills_list;
 mod thread_archive;
 mod thread_fork;