Auth.getCurrentUser returns user sub even if user logged in with email

### Before opening, please confirm:

- [x] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-android/issues?q=is%3Aissue+) and [discussions](https://github.com/aws-amplify/amplify-android/discussions).

### Language and Async Model

Java

### Amplify Categories

Authentication

### Gradle script dependencies

```
def aws_amplify_version = '2.30.1'
coreLibraryDesugaring("com.android.tools:desugar_jdk_libs:2.1.5")
implementation "com.amplifyframework:aws-api:$aws_amplify_version"
implementation "com.amplifyframework:aws-auth-cognito:$aws_amplify_version"
implementation "com.amplifyframework:core:$aws_amplify_version"
```

### Environment information

```
------------------------------------------------------------
Gradle 8.14.2
------------------------------------------------------------

Build time: 2025-06-05 13:32:01 UTC
Revision: 30db2a3bdfffa9f8b40e798095675f9dab990a9a

Kotlin: 2.0.21
Groovy: 3.0.24
Ant: Apache Ant(TM) version 1.10.15 compiled on August 25 2024
Launcher JVM: 17.0.12 (Oracle Corporation 17.0.12+8-LTS-286)
Daemon JVM: E:\Programare\jdk-17.0.12 (no JDK specified, using current Java home)
OS: Windows 11 10.0 amd64
```

### Please include any relevant guides or documentation you're referencing

_No response_

### Describe the bug

Auth.getCurrentUser returns user sub even if user logged in with email.

The AWS Mobile Client was returning the email in such cases. The javadoc said:

```
/**
* Returns the username attribute of the current access token. Note that the value stored in the username
* attribute of the access token may vary depending on how sign-in was performed. For example, if the user signed in
* with email, the username attribute will have the email address.
* @return The username attribute of the current access token.
* @see Using the Access Token
* from Cognito documentation.
*/
@AnyThread
public String getUsername() {
try {
if (userpoolsLoginKey.equals(mStore.get(PROVIDER_KEY))) {
return userpool.getCurrentUser().getUserId();
}
return null;
} catch (Exception e) {
return null;
}
}

```

### Reproduction steps (if applicable)

_No response_

### Code Snippet

```
CompletableFuture future = new CompletableFuture<>();
Amplify.Auth.getCurrentUser(
user -> future.complete(user.getUsername()),
authException -> future.complete(null)
);
```

Both the user id and the username attributes in AuthUser are the user sub, regardless of how the user logged in.

### Log output

```
// Put your logs below this line

```

### Configuration File

_No response_

### GraphQL Schema

```graphql
// Put your schema below this line

```

### Additional information and screenshots

_No response_

Start from the Auth.getUsername() implementation shown in the issue and inspect how the current Cognito user and access-token username are selected. Verify the behavior for email sign-in against the documented expectation, then add or update the relevant authentication test so completion is demonstrated by returning the email rather than the user sub.

Written by the indexing model from the issue text.