[PM-22678] Send email otp authentication method (#6255)

feat(auth): email OTP validation, and generalize authentication interface

- Generalized send authentication method interface
- Made validate method async
- Added email mail support for Handlebars
- Modified email templates to match future implementation

fix(auth): update constants, naming conventions, and error handling

- Renamed constants for clarity
- Updated claims naming convention
- Fixed error message generation
- Added customResponse for Rust consumption

test(auth): add and fix tests for validators and email

- Added tests for SendEmailOtpRequestValidator
- Updated tests for SendAccessGrantValidator

chore: apply dotnet formatting

src/Core/Services/Implementations/HandlebarsMailService.cs

@@ -15,6 +15,7 @@ using Bit.Core.Billing.Models.Mail;
 using Bit.Core.Entities;
 using Bit.Core.Models.Data.Organizations;
 using Bit.Core.Models.Mail;
+using Bit.Core.Models.Mail.Auth;
 using Bit.Core.Models.Mail.Billing;
 using Bit.Core.Models.Mail.FamiliesForEnterprise;
 using Bit.Core.Models.Mail.Provider;
@@ -199,6 +200,26 @@ public class HandlebarsMailService : IMailService
         await _mailDeliveryService.SendEmailAsync(message);
     }
 
+    public async Task SendSendEmailOtpEmailAsync(string email, string token, string subject)
+    {
+        var message = CreateDefaultMessage(subject, email);
+        var requestDateTime = DateTime.UtcNow;
+        var model = new DefaultEmailOtpViewModel
+        {
+            Token = token,
+            TheDate = requestDateTime.ToLongDateString(),
+            TheTime = requestDateTime.ToShortTimeString(),
+            TimeZone = _utcTimeZoneDisplay,
+            WebVaultUrl = _globalSettings.BaseServiceUri.VaultWithHash,
+            SiteName = _globalSettings.SiteName,
+        };
+        await AddMessageContentAsync(message, "Auth.SendAccessEmailOtpEmail", model);
+        message.MetaData.Add("SendGridBypassListManagement", true);
+        // TODO - PM-25380 change to string constant
+        message.Category = "SendEmailOtp";
+        await _mailDeliveryService.SendEmailAsync(message);
+    }
+
     public async Task SendFailedTwoFactorAttemptEmailAsync(string email, TwoFactorProviderType failedType, DateTime utcNow, string ip)
     {
         // Check if we've sent this email within the last hour