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/codex-api/src/common.rs

@@ -56,6 +56,9 @@ pub enum ResponseEvent {
     Created,
     OutputItemDone(ResponseItem),
     OutputItemAdded(ResponseItem),
+    /// Emitted when the server includes `OpenAI-Model` on the stream response.
+    /// This can differ from the requested model when backend safety routing applies.
+    ServerModel(String),
     /// Emitted when `X-Reasoning-Included: true` is present on the response,
     /// meaning the server already accounted for past reasoning tokens and the
     /// client should not re-estimate them.