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/app-server-protocol/schema/json/ApplyPatchApprovalResponse.json

@@ -88,11 +88,26 @@
           "type": "object"
         },
         {
+          "additionalProperties": false,
           "description": "User has denied this command and the agent should not execute it, but it should continue the session and try something else.",
-          "enum": [
+          "properties": {
+            "denied": {
+              "properties": {
+                "rejection": {
+                  "type": "string"
+                }
+              },
+              "required": [
+                "rejection"
+              ],
+              "type": "object"
+            }
+          },
+          "required": [
             "denied"
           ],
-          "type": "string"
+          "title": "DeniedReviewDecision",
+          "type": "object"
         },
         {
           "description": "Automatic approval review timed out before reaching a decision.",