feat: implement Esendex SMS provider (UK / Europe) #25

Summary

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

Esendex is a UK-based business messaging provider with strong coverage in the UK, Europe, and Australia.

Files to create

• src/SmsBridge/Providers/Esendex/EsendexSmsProvider.cs
• src/SmsBridge/Providers/Esendex/EsendexSmsRequestMapper.cs
• src/SmsBridge/Providers/Esendex/EsendexSmsResponseMapper.cs
• src/SmsBridge/Options/EsendexOptions.cs
• Register UseEsendex() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.Esendex in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.Esendex to the enum

Esendex API Details

• Endpoint: https://api.esendex.com/v1.0/messagedispatcher
• Auth: HTTP Basic Auth (user@email:password)
• Request (XML):

  <messages>
    <accountreference>...</accountreference>
    <message>
      <to>...</to>
      <body>...</body>
    </message>
  </messages>

• Response (XML): <messageheaders batchid="..."><header uri="..." id="..." /></messageheaders>

Note: Esendex uses XML for both request and response bodies. Use System.Xml.Linq — do not add an XML serialisation package.

Options

public sealed class EsendexOptions
{
    public required string Username { get; init; }
    public required string Password { get; init; }
    public required string AccountReference { get; init; }
    public string? From { get; init; }
}

Tests to add

• tests/SmsBridge.ProviderTests/EsendexResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/EsendexProviderTests.cs

Acceptance Criteria

• Provider sends via Esendex REST API using IHttpClientFactory
• Request is XML (Content-Type: application/xml)
• Response mapper parses XML and extracts the message header id
• Transient failures set IsTransientFailure = true
• UseEsendex() registers the provider in DI
• All tests pass — dotnet test
• dotnet build has 0 warnings