feat: implement LINK Mobility SMS provider (Europe / Nordics) #24

Summary

Implement the LINK Mobility SMS provider inside src/SmsBridge/Providers/LinkMobility/.

LINK Mobility is a Norwegian company and the largest messaging provider in the Nordic region, with broad European coverage.

Files to create

• src/SmsBridge/Providers/LinkMobility/LinkMobilitySmsProvider.cs
• src/SmsBridge/Providers/LinkMobility/LinkMobilitySmsRequestMapper.cs
• src/SmsBridge/Providers/LinkMobility/LinkMobilitySmsResponseMapper.cs
• src/SmsBridge/Options/LinkMobilityOptions.cs
• Register UseLinkMobility() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.LinkMobility in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.LinkMobility to the enum

LINK Mobility API Details

• Endpoint: https://wsx.sp247.net/sms/send
• Auth: Authorization: Basic <base64(username:password)>
• Request (JSON):

  {
    "platformId": "...",
    "platformPartnerId": "...",
    "useDeliveryReport": false,
    "sendRequestMessages": [{
      "source": "...",
      "destination": "...",
      "userData": "..."
    }]
  }

• Response: { "resultCode": "1001", "description": "Queued" }

Options

public sealed class LinkMobilityOptions
{
    public required string Username { get; init; }
    public required string Password { get; init; }
    public required string PlatformId { get; init; }
    public required string PlatformPartnerId { get; init; }
    public required string From { get; init; }
}

Tests to add

• tests/SmsBridge.ProviderTests/LinkMobilityResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/LinkMobilityProviderTests.cs

Acceptance Criteria

• Provider sends via LINK Mobility REST API using IHttpClientFactory
• Response mapper checks resultCode == "1001" for success
• Transient failures set IsTransientFailure = true
• UseLinkMobility() registers the provider in DI
• All tests pass — dotnet test
• dotnet build has 0 warnings