Add tests verifying custom HTTPClient with proxy configuration

[Copilot]

Issue claimed custom HTTPClient with proxy settings is ignored when passed via ClientParams. Investigation confirms this is not a bug - the implementation correctly uses custom HTTPClient throughout the request chain.

Changes

Added comprehensive test coverage (3 test files, ~400 LOC):

Unit tests (client/custom_httpclient_test.go)

• Custom transport invocation during requests
• HTTPClient flow through RequestHandler
• Default HTTPClient lazy initialization

Integration tests (twilio_custom_httpclient_integration_test.go)

• Full RestClient → RequestHandler → Client chain preservation
• SetTimeout behavior with custom HTTPClient

Proxy scenario tests (twilio_proxy_test.go)

• Exact issue scenario: http.ProxyURL configuration
• Proxy function invocation verification
• Proxy URL preservation through client chain

Example Usage (from tests)

// Pattern from official docs - works correctly
httpClient := &http.Client{
    Transport: &http.Transport{
        Proxy: http.ProxyURL(proxyURL),
    },
}

baseClient := &client.Client{
    Credentials: client.NewCredentials(accountSid, authToken),
    HTTPClient:  httpClient,  // ✓ Properly used for all requests
}
baseClient.SetAccountSid(accountSid)

twilioClient := twilio.NewRestClientWithParams(twilio.ClientParams{
    Client: baseClient,
})

No production code changes required. Tests serve as regression protection and usage documentation.

Warning

GitHub's Secret Scanning detected potential secrets in the added content. We attempted to remediate the issue by updating the code or redacting.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Custom HTTPClient with proxy is ignored when passed via ClientParams</issue_title>
<issue_description>When providing a custom *client.Client with a configured HTTPClient (e.g., with proxy settings) via twilio.NewRestClientWithParams(), the SDK ignores the custom HTTPClient for actual API requests.

Steps to reproduce

// Create http.Client with proxy
httpClient := &http.Client{
    Transport: &http.Transport{
        Proxy: http.ProxyURL(proxyURL),
    },
}

// Create Twilio client with custom HTTPClient
baseClient := &client.Client{
    Credentials: client.NewCredentials(accountSid, authToken),
    HTTPClient:  httpClient,
}
baseClient.SetAccountSid(accountSid)

twilioClient := twilio.NewRestClientWithParams(twilio.ClientParams{
    Client: baseClient,
})

// Make API call - this SHOULD go through proxy but doesn't
twilioClient.Api.FetchBalance(&api.FetchBalanceParams{})

Expected behavior

All HTTP requests should use the provided HTTPClient with proxy configuration.

Actual behavior

Requests bypass the custom HTTPClient and go directly to Twilio API without proxy.

Root cause

In client/client.go, the SendRequest method calls c.doWithErr(req) at line 220. When a custom client implementing BaseClient interface is passed, the SDK's internal request flow doesn't properly use the HTTPClient field from the provided client.

Workaround

Create a wrapper type that implements BaseClient and overrides SendRequest to directly use HTTPClient.Do(req).

Environment

• twilio-go version: v1.25.1 (also verified in v1.28.8 - same issue)
• Go version: 1.21+

Note

The official documentation shows this exact pattern for proxy configuration, but it doesn't work as documented.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Custom HTTPClient with proxy is ignored when passed via ClientParams #309

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.