Propagate approval rejection reasons (#34400)

## What changed

- Change `ReviewDecision::Denied` to carry a rejection string and update the generated protocol schemas.
- Preserve specific rejection reasons through command, patch, network, MCP, delegated, and automatic approval flows so tool results can return them to the model.
- Distinguish invalid approval responses from user declines and truncate oversized rejection messages before adding them to model context.

## Testing

- Add coverage for denial serialization, invalid app-server approval responses, dropped automatic reviews, network rejection propagation, and rejection-message truncation.

GitOrigin-RevId: d48a55643aa03c7ecfa08a8e4b669913eec5464a

codex-rs/mcp-server/src/exec_approval.rs

@@ -130,7 +130,7 @@ async fn on_exec_approval_response(
         // If we cannot deserialize the response, we deny the request to be
         // conservative.
         ExecApprovalResponse {
-            decision: ReviewDecision::Denied,
+            decision: ReviewDecision::denied("approval request failed"),
         }
     });
 

codex-rs/mcp-server/src/patch_approval.rs

@@ -113,7 +113,7 @@ pub(crate) async fn on_patch_approval_response(
             if let Err(submit_err) = codex
                 .submit(Op::PatchApproval {
                     id: approval_id.clone(),
-                    decision: ReviewDecision::Denied,
+                    decision: ReviewDecision::denied("approval request failed"),
                 })
                 .await
             {
@@ -126,7 +126,7 @@ pub(crate) async fn on_patch_approval_response(
     let response = serde_json::from_value::<PatchApprovalResponse>(value).unwrap_or_else(|err| {
         error!("failed to deserialize PatchApprovalResponse: {err}");
         PatchApprovalResponse {
-            decision: ReviewDecision::Denied,
+            decision: ReviewDecision::denied("approval request failed"),
         }
     });