feat: implement Beem SMS provider (Tanzania / East Africa) #56

Summary

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

Beem is a Tanzania/East Africa communication provider with public API documentation covering SMS, two-way SMS, multicountry SMS/SMPP, OTP, reporting, and related services. This gives SMSBridge stronger East African coverage.

Official docs:

• API docs: https://docs.beem.africa/

Files to create

• src/SmsBridge/Providers/Beem/BeemSmsProvider.cs
• src/SmsBridge/Providers/Beem/BeemSmsRequestMapper.cs
• src/SmsBridge/Providers/Beem/BeemSmsResponseMapper.cs
• src/SmsBridge/Options/BeemOptions.cs
• Register UseBeem() in src/SmsBridge/DependencyInjection/SmsBridgeProviderBuilder.cs
• Add HttpClientNames.Beem in src/SmsBridge/Internal/Http/HttpClientNames.cs
• Add SmsProviderType.Beem to the enum

Beem API Details

• Docs include SMS, two-way SMS, multicountry SMS/SMPP, OTP, and reporting sections.
• Auth is API key/secret based.
• Confirm the current send endpoint and payload from the Beem SMS docs during implementation.
• Keep initial scope to single-recipient outbound SMS.

Options

public sealed class BeemOptions
{
    public required string ApiKey { get; init; }
    public required string SecretKey { get; init; }
    public required string SenderId { get; init; }
}

Implementation Notes

• Use the public Beem docs to confirm whether auth is HTTP Basic, token-based, or custom header-based before coding.
• Map the Beem request id/message id to SmsSendResult.ProviderMessageId.
• Preserve Beem status/error details in failure messages.
• Keep OTP and reporting outside the initial provider implementation.

Tests to add

• tests/SmsBridge.ProviderTests/BeemResponseMapperTests.cs
• tests/SmsBridge.ProviderTests/BeemProviderTests.cs

Acceptance Criteria

• Provider sends SMS through Beem using IHttpClientFactory
• Provider authenticates with configured API credentials
• Request mapper emits sender, recipient, and message content
• Response mapper maps provider id/reference to SmsSendResult.ProviderMessageId
• Provider maps Beem API failures to failed SmsSendResult
• Transient HTTP failures set IsTransientFailure = true
• UseBeem() registers the provider in DI
• All tests pass - dotnet test
• dotnet build has 0 warnings