Fix shallow Request clone sharing query/headers/config for pooled sends

[JonPurvis]

Summary

Fixes #524.

Request uses lazy-initialized object stores (query, headers, config, delay, middlewarePipeline). PHP’s default clone is shallow, so if those stores are initialized before cloning, clones share the same instances. Concurrent pools (e.g. async pagination) then mutate the same query bag and requests can send the wrong page (or other params).

Changes

• Add Request::__clone() to deep-clone any initialized mutable stores.
• Add MiddlewarePipeline::__clone() to clone internal Pipeline instances so middleware state is not shared across cloned requests.

Before

Runs: 10 | Pages: 10 | Concurrency: 5

Run 1: [1,6,6,6,6,7,6,8,10,10] (unique=5/10)
Run 2: [1,6,6,6,6,6,7,8,10,9] (unique=6/10)
Run 3: [1,6,6,6,6,6,7,8,9,10] (unique=6/10)
Run 4: [1,6,6,6,6,6,7,8,9,10] (unique=6/10)
Run 5: [1,6,6,6,6,7,6,8,9,10] (unique=6/10)
Run 6: [1,6,6,6,6,6,7,8,9,10] (unique=6/10)
Run 7: [1,6,6,6,6,6,7,8,9,10] (unique=6/10)
Run 8: [1,6,6,6,6,6,8,9,8,10] (unique=5/10)
Run 9: [1,6,6,6,6,7,6,8,9,10] (unique=6/10)
Run 10: [1,6,6,6,6,6,7,8,9,10] (unique=6/10)

After

Runs: 10 | Pages: 10 | Concurrency: 5

Run 1: [1,3,4,2,5,7,6,8,9,10] (unique=10/10)
Run 2: [1,2,3,4,6,5,8,7,9,10] (unique=10/10)
Run 3: [1,2,4,3,5,6,7,8,9,10] (unique=10/10)
Run 4: [1,2,3,4,6,5,7,8,10,9] (unique=10/10)
Run 5: [1,3,5,6,7,2,8,9,10,4] (unique=10/10)
Run 6: [1,6,4,2,5,7,3,9,8,10] (unique=10/10)
Run 7: [1,2,4,5,6,7,3,8,9,10] (unique=10/10)
Run 8: [1,2,6,3,5,7,4,8,9,10] (unique=10/10)
Run 9: [1,2,3,4,5,6,7,8,9,10] (unique=10/10)
Run 10: [1,2,4,6,5,3,9,7,10,8] (unique=10/10)

Testing

• Added regression tests covering:
  • Independent query / headers / config / delay after clone
  • Independent middleware pipelines after clone
  • Concurrent pool() with multiple clones + distinct page query values