Fix httputil.ResponseChain deadlock

[dwisiswant0]

Changes:

• sizedpool:

  feat(sizedpool): add `Discard` method to release semaphore slot
  
  Add `(*SizedPool[T]).Discard` method that releases the semaphore
  slot without returning the item to the underlying
  [sync.Pool]. This enables callers to properly
  release resources when intentionally discarding
  items (e.g., oversized buffers) instead of
  returning them to the pool.
  
  Also, refactor `(*SizedPool[T]).Put()` to use
  `(*SizedPool[T]).Discard()` internally.
  

• httputil:

  fix(httputil): semaphore leak causing deadlock in buffer pool
  
  When `putBuffer()` discarded buffers (either
  oversized or when the large buffer channel was
  full), the sizedpool semaphore slot acquired by
  `getBuffer()` was never released. This caused
  semaphore exhaustion and eventual deadlock under
  sustained load with large response bodies.
  
  Call `bufPool.Discard()` in both discard paths to
  properly release the semaphore slot.
  

Proof

patch:

$ go test -race -run ^TestSemaphoreLeakDeadlock$ ./http
ok  	github.com/projectdiscovery/utils/http	3.451s

main:

$ git cherry-pick fcf5221
[main 8a60f1f] test(httputil): add TestSemaphoreLeakDeadlock
 Date: Fri Dec 19 20:47:49 2025 +0700
 1 file changed, 68 insertions(+)
$ go test -race -run ^TestSemaphoreLeakDeadlock$ ./http
--- FAIL: TestSemaphoreLeakDeadlock (1.24s)
    respChain_test.go:1267: iteration 1, response 0
    respChain_test.go:1267: iteration 1, response 1
    respChain_test.go:1267: iteration 2, response 0
    respChain_test.go:1267: iteration 2, response 1
    respChain_test.go:1267: iteration 3, response 0
    respChain_test.go:1267: iteration 3, response 1
    respChain_test.go:1267: iteration 4, response 0
    respChain_test.go:1267: iteration 4, response 1
    respChain_test.go:1267: iteration 5, response 0
    respChain_test.go:1267: iteration 5, response 1
    respChain_test.go:1267: iteration 6, response 0
    respChain_test.go:1267: iteration 6, response 1
    respChain_test.go:1267: iteration 7, response 0
    respChain_test.go:1267: iteration 7, response 1
    respChain_test.go:1267: iteration 8, response 0
    respChain_test.go:1267: iteration 8, response 1
    respChain_test.go:1267: iteration 9, response 0
    respChain_test.go:1267: iteration 9, response 1
    respChain_test.go:1267: iteration 10, response 0
    respChain_test.go:1267: iteration 10, response 1
    respChain_test.go:1267: iteration 11, response 0
    respChain_test.go:1267: iteration 11, response 1
    respChain_test.go:1291: Deadlock detected at iteration 11
FAIL
FAIL	github.com/projectdiscovery/utils/http	1.274s
FAIL