Private/Get-GraphErrorMessage.ps1

function Get-GraphErrorMessage {
    <#
    .SYNOPSIS
        Extracts error message from Graph API error response
    .DESCRIPTION
        Internal helper function for parsing Graph API error details
    #>

    [CmdletBinding()]
    param(
        [Parameter(Mandatory = $true)]
        [System.Management.Automation.ErrorRecord]$ErrorRecord
    )
    if ($ErrorRecord.ErrorDetails -and $ErrorRecord.ErrorDetails.Message) {
        return $ErrorRecord.ErrorDetails.Message
    }
    return $ErrorRecord.Exception.Message
}