test method for the getContentDocumentBase64 method in Apex

@isTest
static void testGetContentDocumentBase64() {
    // Create a test ContentVersion
    Blob bodyBlob = Blob.valueOf('Test File Content');
    ContentVersion cv = new ContentVersion();
    cv.Title = 'TestFile';
    cv.PathOnClient = 'TestFile.jpg';
    cv.VersionData = bodyBlob;
    insert cv;

    // Query ContentDocumentId from inserted ContentVersion
    ContentVersion insertedCV = [SELECT ContentDocumentId FROM ContentVersion WHERE Id = :cv.Id LIMIT 1];

    // Call the method with the ContentDocumentId
    List<String> result = YourClassName.getContentDocumentBase64(new List<String>{insertedCV.ContentDocumentId});

    // Verify the result is not null or empty
    System.assertNotEquals(null, result);
    System.assert(!result.isEmpty());
}