Behavior: Updates the authenticated user's profile information including FirstName, LastName, and custom Properties. Users can only update their own profile — the Id in the request must match the authenticated user.

Authorization: Requires a valid Bearer token (class-level [Authorize]). Users can only update their own profile.

Side Effects: Updates the user's profile information.

Error Scenarios: Returns 400 if the request is null. Returns 404 if the user is not found. Throws if the authenticated user attempts to update a different user's profile.