[Codex][CLI] Show model-capacity guidance on 429 (#10118)

###### Problem
Users get generic 429s with no guidance when a model is at capacity.
###### Solution
Detect model-cap headers, surface a clear “try a different model”
message, and keep behavior non‑intrusive (no auto‑switch).
###### Scope
CLI/TUI only; protocol + error mapping updated to carry model‑cap info.
###### Tests
      - just fmt
      - cargo test -p codex-tui
- cargo test -p codex-core --lib
shell_snapshot::tests::try_new_creates_and_deletes_snapshot_file --
--nocapture (ran in isolated env)
      - validate local build with backend
     
<img width="719" height="845" alt="image"
src="https://github.com/user-attachments/assets/1470b33d-0974-4b1f-b8e6-d11f892f4b54"
/>

codex-rs/protocol/src/protocol.rs

@@ -925,6 +925,10 @@ pub enum AgentStatus {
 pub enum CodexErrorInfo {
     ContextWindowExceeded,
     UsageLimitExceeded,
+    ModelCap {
+        model: String,
+        reset_after_seconds: Option<u64>,
+    },
     HttpConnectionFailed {
         http_status_code: Option<u16>,
     },