fix(iam): fromUserArn returns incorrect principalAccount

[pahud]

Issue # (if applicable)

Closes #29999

Reason for this change

As described in the issue 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

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

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

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

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[aaythapa]

LGTM, +1 to nmussy@ comment (will approve after its addressed)

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[aaythapa]

@mergify update

[mergify]

update

✅ Branch has been successfully updated

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 28b51e5
• Result: SUCCEEDED
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