fix(iam): fromUserArn returns incorrect principalAccount (#30023)

### Issue # (if applicable)

Closes #29999

### Reason for this change

As described in the issue [comment](#29999 (comment)).

### Description of changes



### Description of how you validated changes

1. added more unit tests.
2. added a new integ test
3. I have deployed this in my AWS account

```ts
import {
  App, Stack, CfnParameter,
  aws_iam as iam,
  CfnOutput,
} from 'aws-cdk-lib';

const app = new App();
const stack = new Stack(app, 'dummy-stack');

const userArn = 'arn:aws:iam::123456789012:user/OthersExternalIamUser';

const userparam = new CfnParameter(stack, 'UserParameter', {
  default: userArn,
});

const imported = iam.User.fromUserArn(stack, 'imported-user', userArn);
const imported2 = iam.User.fromUserArn(stack, 'imported-user2', userparam.valueAsString );

new CfnOutput(stack, 'User', { value: imported.principalAccount! });
new CfnOutput(stack, 'User2', { value: imported2.principalAccount! });
```
And the output is correct:

```
Outputs:
dummy-stack.User = 123456789012
dummy-stack.User2 = 123456789012
```




### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: pahud

packages/@aws-cdk-testing/framework-integ/test/aws-iam/test/integ.user-import.js.snapshot/cdk.out

@@ -0,0 +1,83 @@
+{
+ "Parameters": {
+  "UserParameter": {
+   "Type": "String",
+   "Default": "arn:aws:iam::123456789012:user/OthersExternalIamUser"
+  },
+  "BootstrapVersion": {
+   "Type": "AWS::SSM::Parameter::Value<String>",
+   "Default": "/cdk-bootstrap/hnb659fds/version",
+   "Description": "Version of the CDK Bootstrap resources in this environment, automatically retrieved from SSM Parameter Store. [cdk:skip]"
+  }
+ },
+ "Resources": {
+  "User00B015A1": {
+   "Type": "AWS::IAM::User"
+  }
+ },
+ "Outputs": {
+  "UserOutput": {
+   "Value": "123456789012"
+  },
+  "User2Output": {
+   "Value": {
+    "Fn::Select": [
+     4,
+     {
+      "Fn::Split": [
+       ":",
+       {
+        "Ref": "UserParameter"
+       }
+      ]
+     }
+    ]
+   }
+  },
+  "User3Output": {
+   "Value": {
+    "Fn::Select": [
+     4,
+     {
+      "Fn::Split": [
+       ":",
+       {
+        "Fn::GetAtt": [
+         "User00B015A1",
+         "Arn"
+        ]
+       }
+      ]
+     }
+    ]
+   }
+  }
+ },
+ "Rules": {
+  "CheckBootstrapVersion": {
+   "Assertions": [
+    {
+     "Assert": {
+      "Fn::Not": [
+       {
+        "Fn::Contains": [
+         [
+          "1",
+          "2",
+          "3",
+          "4",
+          "5"
+         ],
+         {
+          "Ref": "BootstrapVersion"
+         }
+        ]
+       }
+      ]
+     },
+     "AssertDescription": "CDK bootstrap stack version 6 required. Please run 'cdk bootstrap' with a recent version of the CDK CLI."
+    }
+   ]
+  }
+ }
+}