feat(OTP): [PM-18612] Change email OTP to six digits

* Change email OTP to 6 digits

* Added comment on base class

* Added tests

* Renamed tests.

* Fixed tests

* Renamed file to match class

test/Core.Test/Auth/Identity/EmailTokenProviderTests.cs

@@ -0,0 +1,68 @@
+using Bit.Core;
+using Bit.Core.Auth.Identity.TokenProviders;
+using Bit.Core.Entities;
+using Bit.Core.Services;
+using Bit.Test.Common.AutoFixture.Attributes;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.Extensions.Caching.Distributed;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using NSubstitute;
+using Xunit;
+
+[SutProviderCustomize]
+public class EmailTokenProviderTests
+{
+    private readonly IDistributedCache _cache;
+
+    public EmailTokenProviderTests()
+    {
+        _cache = Substitute.For<IDistributedCache>();
+    }
+
+    [Theory]
+    [BitAutoData]
+    public async Task GenerateAsync_GeneratesSixDigitToken_WhenFeatureFlagIsEnabled(User user)
+    {
+        // Arrange
+        var purpose = "test-purpose";
+        var featureService = Substitute.For<IFeatureService>();
+        featureService.IsEnabled(FeatureFlagKeys.Otp6Digits).Returns(true);
+        var tokenProvider = new EmailTokenProvider(_cache, featureService);
+
+        // Act
+        var code = await tokenProvider.GenerateAsync(purpose, SubstituteUserManager(), user);
+
+        // Assert
+        Assert.Equal(6, code.Length);
+    }
+
+    [Theory]
+    [BitAutoData]
+    public async Task GenerateAsync_GeneratesEightDigitToken_WhenFeatureFlagIsDisabled(User user)
+    {
+        // Arrange
+        var purpose = "test-purpose";
+        var featureService = Substitute.For<IFeatureService>();
+        featureService.IsEnabled(FeatureFlagKeys.Otp6Digits).Returns(false);
+        var tokenProvider = new EmailTokenProvider(_cache, featureService);
+        // Act
+        var code = await tokenProvider.GenerateAsync(purpose, SubstituteUserManager(), user);
+
+        // Assert
+        Assert.Equal(8, code.Length);
+    }
+
+    protected static UserManager<User> SubstituteUserManager()
+    {
+        return new UserManager<User>(Substitute.For<IUserStore<User>>(),
+            Substitute.For<IOptions<IdentityOptions>>(),
+            Substitute.For<IPasswordHasher<User>>(),
+            Enumerable.Empty<IUserValidator<User>>(),
+            Enumerable.Empty<IPasswordValidator<User>>(),
+            Substitute.For<ILookupNormalizer>(),
+            Substitute.For<IdentityErrorDescriber>(),
+            Substitute.For<IServiceProvider>(),
+            Substitute.For<ILogger<UserManager<User>>>());
+    }
+}