feat: implement Textmagic SMS provider (UK / Global) #26

Summary

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

Textmagic is a UK-based business SMS platform with global reach. It is commonly used by small and medium businesses in the UK and Europe.

Files to create

• src/SmsBridge/Providers/Textmagic/TextmagicSmsProvider.cs
• src/SmsBridge/Providers/Textmagic/TextmagicSmsRequestMapper.cs
• src/SmsBridge/Providers/Textmagic/TextmagicSmsResponseMapper.cs
• src/SmsBridge/Options/TextmagicOptions.cs
• Register UseTextmagic() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.Textmagic in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.Textmagic to the enum

Textmagic API Details

• Endpoint: https://rest.textmagic.com/api/v2/messages
• Auth: X-TM-Username and X-TM-Key headers
• Request (JSON):

  { "text": "...", "phones": "...", "from": "..." }

• Response: { "id": ..., "href": "...", "type": "message", "sessionId": ... }

Options

public sealed class TextmagicOptions
{
    public required string Username { get; init; }
    public required string ApiKey { get; init; }
    public required string From { get; init; }
}

Tests to add

• tests/SmsBridge.ProviderTests/TextmagicResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/TextmagicProviderTests.cs

Acceptance Criteria

• Provider sends via Textmagic REST API using IHttpClientFactory
• X-TM-Username and X-TM-Key headers are set correctly
• Response mapper extracts id as the provider message ID
• Transient failures set IsTransientFailure = true
• UseTextmagic() registers the provider in DI
• All tests pass — dotnet test
• dotnet build has 0 warnings