feat: implement Telesign SMS provider (US / Global) #54

Summary

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

Telesign is a US/global identity and communications provider with SMS messaging APIs. This adds another established US/global provider for verification and transactional messaging use cases.

Official docs:

• Developer docs: https://developer.telesign.com/
• Enterprise SMS docs: https://developer.telesign.com/enterprise

Files to create

• src/SmsBridge/Providers/Telesign/TelesignSmsProvider.cs
• src/SmsBridge/Providers/Telesign/TelesignSmsRequestMapper.cs
• src/SmsBridge/Providers/Telesign/TelesignSmsResponseMapper.cs
• src/SmsBridge/Options/TelesignOptions.cs
• Register UseTelesign() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.Telesign in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.Telesign to the enum

Telesign API Details

• Messaging endpoint commonly documented as: POST https://rest-ww.telesign.com/v1/messaging
• Auth: Customer ID plus API key using Telesign request signing
• Request fields commonly include:

  phone_number=15557654321
  message=Hello from SMSBridge
  message_type=ARN
  

• Response includes a provider reference id and status/error details.

Options

public sealed class TelesignOptions
{
    public required string CustomerId { get; init; }
    public required string ApiKey { get; init; }
    public string MessageType { get; init; } = "ARN";
}

Implementation Notes

• Confirm the current Telesign signing requirements during implementation; their docs are hosted behind a dynamic ReadMe site and the exact endpoint page should be rechecked before coding.
• Keep the request signing helper private to the Telesign provider unless another provider needs the same scheme.
• Map the provider reference id to SmsSendResult.ProviderMessageId.
• Map Telesign status/error payloads into SMSBridge success/failure semantics.

Tests to add

• tests/SmsBridge.ProviderTests/TelesignResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/TelesignProviderTests.cs
• Unit tests for Telesign request signing

Acceptance Criteria

• Provider signs Telesign requests using CustomerId and ApiKey
• Provider sends via the current Telesign SMS/messaging endpoint using IHttpClientFactory
• Request mapper emits phone_number, message, and message_type
• Response mapper maps provider reference id to SmsSendResult.ProviderMessageId
• Provider maps Telesign status/error responses to failed SmsSendResult
• Transient HTTP failures set IsTransientFailure = true
• UseTelesign() registers the provider in DI
• All tests pass - dotnet test
• dotnet build has 0 warnings