Add HTTPS Test Coverage for HTTP Agent Pooling

[bliuchak]

Problem

PR #619 introduced useful HTTP agent pooling functionality, but the current test suite lacks adequate coverage for HTTPS upstreams.

• We currently have limited tooling for comprehensive HTTPS testing, making it difficult to implement full coverage immediately.
• This limitation may be mitigated after the release of https://github.com/apify/proxy-chain/pull/602, which is expected to simplify our testing capabilities.

Given the dependency and the good-enough coverage for HTTP, we decided to merge #619 as is, while acknowledging the insufficient coverage for HTTPS.

Proposed Solution

• Conduct a test coverage audit focused specifically on the HTTP/HTTPS agent pooling feature.
• Identify and implement critical missing tests (primarly for HTTPS upstreams).

Refs

• https://github.com/apify/proxy-chain/pull/619#issuecomment-3532578371