Implement native HttpClient MultiCurlAdapter for multi requests

[armanist]

Parent

Part of #565.

Summary

Replace the vendor-backed Quantum\HttpClient\Adapters\MultiCurlAdapter internals with a native multi-curl implementation while preserving existing multi-request and async callback behavior.

This ticket should be handled separately from the single-request adapter because multi-curl behavior is more complex and carries higher regression risk.

Scope

• Implement native multi-curl execution behind MultiCurlAdapter.
• Preserve current public HttpClient multi-request behavior:
  • createMultiRequest()
  • createAsyncMultiRequest()
  • addGet()
  • addPost()
  • setHeader() / setHeaders()
  • setOpt() / setOpts()
  • start()
  • getResponse()
  • getErrors()
  • isMultiRequest() guard behavior
• Preserve factory/helper usage:
  • HttpClientFactory::createMultiRequest()
  • HttpClientFactory::createAsyncMultiRequest($success, $error)
  • httpMultiRequest()
  • httpAsyncMultiRequest($success, $error)

Behavior To Match

• Queueing multiple GET and POST requests.
• Returning Quantum CurlAdapter instances from addGet() and addPost() where current adapter behavior does so.
• Complete callback registration and invocation.
• Success callback registration and invocation.
• Error callback registration and invocation.
• Response aggregation by request ID.
• Error aggregation by request ID.
• Request ID stability for queued requests.
• Header and option propagation to queued requests.
• Response headers, cookies, and body shape expected by HttpClient.
• Async callback behavior without leaking raw vendor/native handles across the adapter boundary.

Acceptance Criteria

• MultiCurlAdapter no longer depends on Curl\MultiCurl for normal execution.
• Existing multi-request and async HttpClient tests pass.
• Focused tests cover queueing, callbacks, success/error behavior, response aggregation, and option/header propagation.
• Downstream or local smoke coverage confirms multi-request execution where practical.
• After this ticket and the single-request native adapter ticket are complete, evaluate and remove php-curl-class/php-curl-class from Composer if no compatibility bridge remains.

Out Of Scope

• Changing the public HttpClient facade API.
• Reworking the factory/helper API.
• Removing php-curl-class/php-curl-class before single and multi native adapter parity is complete.