[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/app-server-protocol/src/protocol/v2.rs

@@ -86,6 +86,10 @@ macro_rules! v2_enum_from_core {
 pub enum CodexErrorInfo {
     ContextWindowExceeded,
     UsageLimitExceeded,
+    ModelCap {
+        model: String,
+        reset_after_seconds: Option<u64>,
+    },
     HttpConnectionFailed {
         #[serde(rename = "httpStatusCode")]
         #[ts(rename = "httpStatusCode")]
@@ -122,6 +126,13 @@ impl From<CoreCodexErrorInfo> for CodexErrorInfo {
         match value {
             CoreCodexErrorInfo::ContextWindowExceeded => CodexErrorInfo::ContextWindowExceeded,
             CoreCodexErrorInfo::UsageLimitExceeded => CodexErrorInfo::UsageLimitExceeded,
+            CoreCodexErrorInfo::ModelCap {
+                model,
+                reset_after_seconds,
+            } => CodexErrorInfo::ModelCap {
+                model,
+                reset_after_seconds,
+            },
             CoreCodexErrorInfo::HttpConnectionFailed { http_status_code } => {
                 CodexErrorInfo::HttpConnectionFailed { http_status_code }
             }