feat: implement seven.io (sms77) SMS provider (Germany / Europe) #27

Summary

Implement the seven.io (formerly sms77) SMS provider inside src/SmsBridge/Providers/SevenIo/.

seven.io is a German SMS gateway with strong coverage in Germany and the DACH region (Germany, Austria, Switzerland), and global reach.

Files to create

• src/SmsBridge/Providers/SevenIo/SevenIoSmsProvider.cs
• src/SmsBridge/Providers/SevenIo/SevenIoSmsRequestMapper.cs
• src/SmsBridge/Providers/SevenIo/SevenIoSmsResponseMapper.cs
• src/SmsBridge/Options/SevenIoOptions.cs
• Register UseSevenIo() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.SevenIo in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.SevenIo to the enum

seven.io API Details

• Endpoint: https://gateway.seven.io/api/sms
• Auth: X-Api-Key header
• Request (form-encoded or JSON):

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

• Response: plain text 100 on success, or an error code on failure
  • 100 = sent successfully
  • 900 = auth error (permanent)
  • 600 = carrier error (transient)

Note: The response is a plain text status code, not JSON. Parse it directly.

Options

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

Tests to add

• tests/SmsBridge.ProviderTests/SevenIoResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/SevenIoProviderTests.cs

Acceptance Criteria

• Provider sends via seven.io REST API using IHttpClientFactory
• Response mapper parses plain-text status code — 100 = success
• Error code 600 sets IsTransientFailure = true; 900 sets false
• UseSevenIo() registers the provider in DI
• All tests pass — dotnet test
• dotnet build has 0 warnings