Implement native HttpClient CurlAdapter for single requests

[armanist]

Parent

Part of #565.

Summary

Replace the vendor-backed Quantum\HttpClient\Adapters\CurlAdapter internals with a native curl implementation for single requests while preserving the existing HttpClient facade behavior and CurlAdapterInterface contract.

Scope

• Implement native single-request curl execution behind CurlAdapter.
• Preserve current public HttpClient methods and behavior:
  • createRequest()
  • setMethod()
  • setData()
  • setHeader() / setHeaders()
  • setOpt() / setOpts()
  • start()
  • getResponseBody()
  • getResponseHeaders()
  • getResponseCookies()
  • getErrors()
  • info()
  • url()
• Preserve factory/helper usage:
  • HttpClientFactory::createRequest($url)
  • httpRequest($url)
• Keep compatibility constructor path if still needed for transition tests, but do not expose native handles as the preferred public API.

Behavior To Match

• Request method handling via CURLOPT_CUSTOMREQUEST.
• Request body handling through existing setData() behavior.
• Header and option forwarding.
• Response body extraction.
• Response header normalization used by HttpClient.
• Cookie extraction shape expected by existing tests.
• Error code/message behavior.
• getInfo() behavior, including option value 0.
• URL reporting through url().

Acceptance Criteria

• CurlAdapter no longer depends on Curl\Curl for normal execution.
• Existing single-request HttpClient behavior remains compatible.
• Focused unit tests cover native curl success, error, headers, cookies, info, URL, options, and payload behavior.
• Downstream remote image download smoke test passes through httpRequest().
• php-curl-class/php-curl-class remains installed until the multi-curl child ticket is complete.

Out Of Scope

• Native multi-curl implementation.
• Removing php-curl-class/php-curl-class from Composer.
• Changing the public HttpClient facade API.