chore(premium): [PM-29186] Remove 2FA user interface from premium method signatures

* Removed 2FA user interface from premium method signatures

* Added some more comments for clarity and small touchups.

* Suggested documentation updates.

---------

Co-authored-by: Patrick Pimentel <ppimentel@bitwarden.com>

src/Core/AdminConsole/Models/Data/Organizations/OrganizationUsers/OrganizationUserUserDetails.cs

@@ -63,11 +63,6 @@ public class OrganizationUserUserDetails : IExternal, ITwoFactorProvidersUser, I
         return UserId;
     }
 
-    public bool GetPremium()
-    {
-        return Premium.GetValueOrDefault(false);
-    }
-
     public Permissions GetPermissions()
     {
         return string.IsNullOrWhiteSpace(Permissions) ? null

src/Core/Auth/Models/ITwoFactorProvidersUser.cs

@@ -1,14 +1,14 @@
-// FIXME: Update this file to be null safe and then delete the line below
-#nullable disable
-
-using Bit.Core.Auth.Enums;
+using Bit.Core.Auth.Enums;
 using Bit.Core.Services;
 
 namespace Bit.Core.Auth.Models;
 
+/// <summary>
+/// An interface representing a user entity that supports two-factor providers
+/// </summary>
 public interface ITwoFactorProvidersUser
 {
-    string TwoFactorProviders { get; }
+    string? TwoFactorProviders { get; }
     /// <summary>
     /// Get the two factor providers for the user. Currently it can be assumed providers are enabled
     /// if they exists in the dictionary. When two factor providers are disabled they are removed
@@ -16,7 +16,10 @@ public interface ITwoFactorProvidersUser
     /// <see cref="IOrganizationService.DisableTwoFactorProviderAsync"/>
     /// </summary>
     /// <returns>Dictionary of providers with the type enum as the key</returns>
-    Dictionary<TwoFactorProviderType, TwoFactorProvider> GetTwoFactorProviders();
+    Dictionary<TwoFactorProviderType, TwoFactorProvider>? GetTwoFactorProviders();
+    /// <summary>
+    /// The unique `UserId` of the user entity for which there are two-factor providers configured.
+    /// </summary>
+    /// <returns>The unique identifier for the user</returns>
     Guid? GetUserId();
-    bool GetPremium();
 }

src/Core/Entities/User.cs

@@ -200,11 +200,6 @@ public class User : ITableObject<Guid>, IStorableSubscriber, IRevisable, ITwoFac
         return Id;
     }
 
-    public bool GetPremium()
-    {
-        return Premium;
-    }
-
     public int GetSecurityVersion()
     {
         // If no security version is set, it is version 1. The minimum initialized version is 2.

src/Core/Services/IUserService.cs

@@ -4,7 +4,6 @@
 using System.Security.Claims;
 using Bit.Core.AdminConsole.Entities;
 using Bit.Core.Auth.Enums;
-using Bit.Core.Auth.Models;
 using Bit.Core.Billing.Models.Business;
 using Bit.Core.Entities;
 using Bit.Core.Enums;
@@ -60,11 +59,22 @@ public interface IUserService
     Task<bool> CheckPasswordAsync(User user, string password);
     /// <summary>
     /// Checks if the user has access to premium features, either through a personal subscription or through an organization.
+    ///
+    /// This is the preferred way to definitively know if a user has access to premium features.
     /// </summary>
     /// <param name="user">user being acted on</param>
     /// <returns>true if they can access premium; false otherwise.</returns>
-    Task<bool> CanAccessPremium(ITwoFactorProvidersUser user);
-    Task<bool> HasPremiumFromOrganization(ITwoFactorProvidersUser user);
+    Task<bool> CanAccessPremium(User user);
+
+    /// <summary>
+    /// Checks if the user has inherited access to premium features through an organization.
+    ///
+    /// This primarily serves as a means to communicate to the client when a user has inherited their premium status
+    /// through an organization. Feature gating logic probably should not be behind this check.
+    /// </summary>
+    /// <param name="user">user being acted on</param>
+    /// <returns>true if they can access premium because of organization membership; false otherwise.</returns>
+    Task<bool> HasPremiumFromOrganization(User user);
     Task<string> GenerateSignInTokenAsync(User user, string purpose);
 
     Task<IdentityResult> UpdatePasswordHash(User user, string newPassword,

src/Core/Services/Implementations/UserService.cs

@@ -1104,7 +1104,7 @@ public class UserService : UserManager<User>, IUserService
         return success;
     }
 
-    public async Task<bool> CanAccessPremium(ITwoFactorProvidersUser user)
+    public async Task<bool> CanAccessPremium(User user)
     {
         var userId = user.GetUserId();
         if (!userId.HasValue)
@@ -1112,10 +1112,10 @@ public class UserService : UserManager<User>, IUserService
             return false;
         }
 
-        return user.GetPremium() || await this.HasPremiumFromOrganization(user);
+        return user.Premium || await HasPremiumFromOrganization(user);
     }
 
-    public async Task<bool> HasPremiumFromOrganization(ITwoFactorProvidersUser user)
+    public async Task<bool> HasPremiumFromOrganization(User user)
     {
         var userId = user.GetUserId();
         if (!userId.HasValue)
@@ -1138,6 +1138,7 @@ public class UserService : UserManager<User>, IUserService
             orgAbility.UsersGetPremium &&
             orgAbility.Enabled);
     }
+
     public async Task<string> GenerateSignInTokenAsync(User user, string purpose)
     {
         var token = await GenerateUserTokenAsync(user, Options.Tokens.PasswordResetTokenProvider,