Added EnsureAndSetUserFields method to ClientContextExtensions

[jeppesc11]

Summary

Adds a new extension method EnsureAndSetUserFields to ClientContext that provides safe assignment of user email addresses to SharePoint list item user fields with automatic user resolution and error handling.

Changes

• Added: EnsureAndSetUserFields extension method to ClientContextExtensions.cs
• Features:
  • Automatic user resolution using ExceptionHandlingScope for robust error handling
  • Supports batch operations for multiple field-email assignments
  • Configurable null/whitespace handling behavior
  • Follows CSOM queuing pattern (requires caller to execute operations)
    ctx.EnsureAndSetUserFields(listItem, fieldAssignments, true);
ctx.ExecuteQueryRetry();

Usage

var fieldAssignments = new[] { ("AssignedTo", "user1@contoso.com"), ("Author", "user2@contoso.com") };
ctx.EnsureAndSetUserFields(listItem, fieldAssignments, clearIfNullOrWhiteSpace: true);
ctx.ExecuteQueryRetry();

Breaking Changes

None - this is a new addition that doesn't affect existing functionality.