Fix partition detection for AssumeRole ARNs

[hugo4715]

Correctly detect and handle AWS partition (aws, aws-cn, aws-us-gov) from role ARNs when assuming roles. This ensures that region selection aligns with the appropriate partition defaults.

Closes #121

Testing done

Tested and validated on Jenkins LTS 2.516.1

Steps done:

• Add an AWS key pair in the jenkins credentials page, in advanced specify an IAM role with a china arn in format "arn:aws-cn:iam::$ACCOUNT:role/$ROLE"
• Observe the credentials page correctly find the key pair is working in china
• Validate the pipeline can get valid credentials from AWS china

The following snippet was used:

withCredentials([aws(accessKeyVariable: 'AWS_ACCESS_KEY_ID', credentialsId: 'jenkins-china', secretKeyVariable: 'AWS_SECRET_ACCESS_KEY')]) {
    sh 'env'
    sh 'aws sts get-caller-identity'
    sh 'aws s3 ls'
}

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed