feat: implement SignalWire SMS provider (US / Global) #53

Summary

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

SignalWire provides SMS/MMS through its compatibility messaging API. This adds another US/global CPaaS provider, especially useful for teams already using SignalWire spaces and phone numbers.

Official docs:

• Messaging docs: https://developer.signalwire.com/messaging/
• Compatibility API docs: https://developer.signalwire.com/compatibility-api/

Files to create

• src/SmsBridge/Providers/SignalWire/SignalWireSmsProvider.cs
• src/SmsBridge/Providers/SignalWire/SignalWireSmsRequestMapper.cs
• src/SmsBridge/Providers/SignalWire/SignalWireSmsResponseMapper.cs
• src/SmsBridge/Options/SignalWireOptions.cs
• Register UseSignalWire() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.SignalWire in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.SignalWire to the enum

SignalWire API Details

• Base URL: https://{SpaceName}.signalwire.com/api/laml/2010-04-01
• Send endpoint: POST /Accounts/{ProjectId}/Messages.json
• Auth: HTTP Basic Auth using Project ID and API token
• Request form fields:

  From=+15551234567
  To=+15557654321
  Body=Hello from SMSBridge
  

• Response is Twilio-compatible and includes fields such as sid, status, to, from, and body.

Options

public sealed class SignalWireOptions
{
    public required string SpaceName { get; init; }
    public required string ProjectId { get; init; }
    public required string ApiToken { get; init; }
    public required string From { get; init; }
}

Implementation Notes

• Use form URL encoded requests, matching compatibility API expectations.
• Use sid as the provider message id for accepted sends.
• The response shape is close to Twilio, but keep a dedicated mapper to avoid coupling provider-specific status behavior.
• Support E.164 numbers.

Tests to add

• tests/SmsBridge.ProviderTests/SignalWireResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/SignalWireProviderTests.cs

Acceptance Criteria

• Provider sends via /Accounts/{ProjectId}/Messages.json using IHttpClientFactory
• Provider uses HTTP Basic Auth with ProjectId:ApiToken
• Request mapper emits From, To, and Body
• Response mapper maps sid to SmsSendResult.ProviderMessageId
• Provider maps rejected statuses/errors to failed SmsSendResult
• Transient HTTP failures set IsTransientFailure = true
• UseSignalWire() registers the provider in DI
• All tests pass - dotnet test
• dotnet build has 0 warnings