feat: implement Cequens SMS provider (MENA) #19

Summary

Implement the Cequens SMS provider inside src/SmsBridge/Providers/Cequens/.

Cequens is an Egypt-based CPaaS platform with strong coverage across the Middle East and North Africa (MENA) region.

Files to create

• src/SmsBridge/Providers/Cequens/CequensSmsProvider.cs
• src/SmsBridge/Providers/Cequens/CequensSmsRequestMapper.cs
• src/SmsBridge/Providers/Cequens/CequensSmsResponseMapper.cs
• src/SmsBridge/Options/CequensOptions.cs
• Register UseCequens() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.Cequens in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.Cequens to the enum

Cequens API Details

• Endpoint: https://apis.cequens.com/sms/v1/messages
• Auth: Authorization: Bearer <api_token>
• Request (JSON):

  {
    "senderName": "...",
    "messageText": "...",
    "messageType": "text",
    "recipients": "..."
  }

• Response: { "messageId": "...", "status": 0 } — status: 0 = success

Options

public sealed class CequensOptions
{
    public required string ApiToken { get; init; }
    public required string From { get; init; }
}

Tests to add

• tests/SmsBridge.ProviderTests/CequensResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/CequensProviderTests.cs

Acceptance Criteria

• Provider sends via Cequens REST API using IHttpClientFactory
• Response mapper checks status == 0 for success
• Transient failures set IsTransientFailure = true
• UseCequens() registers the provider in DI
• All tests pass — dotnet test
• dotnet build has 0 warnings