feat: implement MSG91 SMS provider (India) #21

Summary

Implement the MSG91 SMS provider inside src/SmsBridge/Providers/Msg91/.

MSG91 is one of the leading enterprise SMS and OTP providers in India.

Files to create

• src/SmsBridge/Providers/Msg91/Msg91SmsProvider.cs
• src/SmsBridge/Providers/Msg91/Msg91SmsRequestMapper.cs
• src/SmsBridge/Providers/Msg91/Msg91SmsResponseMapper.cs
• src/SmsBridge/Options/Msg91Options.cs
• Register UseMsg91() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.Msg91 in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.Msg91 to the enum

MSG91 API Details

• Endpoint: https://control.msg91.com/api/v5/flow/
• Auth: authkey header
• Request (JSON):

  {
    "flow_id": "...",
    "sender": "...",
    "mobiles": "...",
    "VAR1": "..."
  }

• Simple SMS (non-template): https://api.msg91.com/api/v5/otp or transactional endpoint
• Response: { "type": "success", "message": "..." }

Note: MSG91 uses template-based flows for most messages in India (due to DLT regulations). Document this limitation clearly in the options.

Options

public sealed class Msg91Options
{
    public required string AuthKey { get; init; }
    public required string From { get; init; }
    public string? TemplateFlowId { get; init; }
}

Tests to add

• tests/SmsBridge.ProviderTests/Msg91ResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/Msg91ProviderTests.cs

Acceptance Criteria

• Provider sends via MSG91 REST API using IHttpClientFactory
• Response mapper checks type == "success"
• Transient failures set IsTransientFailure = true
• DLT/template limitation is documented in Msg91Options XML comments
• UseMsg91() registers the provider in DI
• All tests pass — dotnet test
• dotnet build has 0 warnings