feat: implement FireText SMS provider (UK) #52

Summary

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

FireText is a UK SMS provider with public API documentation for sending SMS, receiving messages, checking credit, scheduled messages, and delivery reports. This adds another UK-local provider alongside Esendex and Textmagic.

Official docs:

• API docs: https://www.firetext.co.uk/docs
• PHP SDK/examples: https://github.com/FireText/FireText-PHP-SDK

Files to create

• src/SmsBridge/Providers/FireText/FireTextSmsProvider.cs
• src/SmsBridge/Providers/FireText/FireTextSmsRequestMapper.cs
• src/SmsBridge/Providers/FireText/FireTextSmsResponseMapper.cs
• src/SmsBridge/Options/FireTextOptions.cs
• Register UseFireText() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.FireText in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.FireText to the enum

FireText API Details

• Base URL: https://www.firetext.co.uk
• Send endpoint: /api/sendsms
• Auth: either apiKey or username/password; prefer apiKey
• Supports GET or POST parameters
• JSON response can be requested by appending /json to API URLs
• Request parameters include apiKey, to, from, and message
• Example endpoint: https://www.firetext.co.uk/api/sendsms/json

Options

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

Implementation Notes

• Prefer POST with form URL encoded parameters to avoid putting message content in query strings.
• Use /api/sendsms/json so response mapping can avoid parsing plain status strings.
• Keep username/password auth out of first implementation unless needed for compatibility.
• Map FireText status numbers and descriptions to success/failure results.

Tests to add

• tests/SmsBridge.ProviderTests/FireTextResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/FireTextProviderTests.cs

Acceptance Criteria

• Provider sends via /api/sendsms/json using IHttpClientFactory
• Request mapper emits apiKey, to, from, and message
• Response mapper handles successful and failed FireText status responses
• Transient HTTP failures set IsTransientFailure = true
• UseFireText() registers the provider in DI
• All tests pass - dotnet test
• dotnet build has 0 warnings