Add config to disable /feedback (#8909)

Some enterprises do not want their users to be able to `/feedback`.

<img width="395" height="325" alt="image"
src="https://github.com/user-attachments/assets/2dae9c0b-20c3-4a15-bcd3-0187857ebbd8"
/>

Adds to `config.toml`:

```toml
[feedback]
enabled = false
```

I've deliberately decided to:
1. leave other references to `/feedback` (e.g. in the interrupt message,
tips of the day) unchanged. I think we should continue to promote the
feature even if it is not usable currently.
2. leave the `/feedback` menu item selectable and display an error
saying it's disabled, rather than remove the menu item (which I believe
would raise more questions).

but happy to discuss these.

This will be followed by a change to requirements.toml that admins can
use to force the value of feedback.enabled.

codex-rs/app-server/src/codex_message_processor.rs

@@ -3678,6 +3678,16 @@ impl CodexMessageProcessor {
     }
 
     async fn upload_feedback(&self, request_id: RequestId, params: FeedbackUploadParams) {
+        if !self.config.feedback_enabled {
+            let error = JSONRPCErrorError {
+                code: INVALID_REQUEST_ERROR_CODE,
+                message: "sending feedback is disabled by configuration".to_string(),
+                data: None,
+            };
+            self.outgoing.send_error(request_id, error).await;
+            return;
+        }
+
         let FeedbackUploadParams {
             classification,
             reason,