Bug Report(Android): prevent NullPointerException crash when Promise.reject receives null code

Hi!

Firstly, thanks for your work on this project!

### Issue Description

**Description**
When calling certain file system methods on Android (e.g., `RNFSManager`), the app crashes on the native side if an exception is thrown.

The underlying cause is that `RNFSManager` passes `null` as the first argument to `promise.reject(null, ex.getMessage())`. Newer versions of React Native implement `PromiseImpl` in Kotlin, where the `code` parameter is strictly defined as non-null (`@NotNull`). Passing `null` causes Kotlin's runtime assertion to throw an unhandled `NullPointerException` on the Android main thread, crashing the application before the error can be handled in JavaScript.

### Environment

- React native 0.83.1
- react-native-fs 2.20

**Stacktrace**
```text
Fatal Exception: java.lang.NullPointerException
Parameter specified as non-null is null: method com.facebook.react.bridge.PromiseImpl.reject, parameter code
at com.facebook.react.bridge.PromiseImpl.reject(PromiseImpl.kt:2)
at com.rnfs.RNFSManager.reject(RNFSManager.java:978)
at com.rnfs.RNFSManager.access$000(RNFSManager.java:60)
```
Here is the diff that solved my problem:

```diff
--- a/node_modules/react-native-fs/android/src/main/java/com/rnfs/RNFSManager.java
+++ b/node_modules/react-native-fs/android/src/main/java/com/rnfs/RNFSManager.java
@@ -975,7 +975,7 @@ public class RNFSManager extends ReactContextBaseJavaModule {
return;
}

- promise.reject(null, ex.getMessage());
+ promise.reject("EUNSPECIFIED", ex.getMessage());
}

private void rejectFileNotFound(Promise promise, String filepath) {

```

Start in android/src/main/java/com/rnfs/RNFSManager.java around line 978, where the stack trace identifies the rejection path, and review the React Native Promise.reject contract in the reported environment. Exercise a failing Android file-system call and confirm that the exception reaches JavaScript as a handled rejection without a native NullPointerException.

Written by the indexing model from the issue text.