fix: keep scanner response resets local and strengthen regression tests

Keep the legacy header resets and clear the unused NSScanner response
field at each construction site. Remove the generic stream Recycler call:
Recycler transfers resource ownership and cannot be treated as a reset
hook followed by another insertion into the handler's pool.

Exercise the actual NSScanner handler across repeated scans with dirty
response allocations, so the regression test does not depend on sync.Pool
reusing an object. Check both response variants after wire serialization.
Cover legacy DecodeMsg and reused indexed metadata headers as well.

Retain the original contributor's commits and DCO sign-offs.

Signed-off-by: Feng Ruohang <rh@vonng.com>

internal/grid/handlers.go

@@ -698,11 +698,6 @@ func (h *StreamTypeHandler[Payload, Req, Resp]) PutRequest(r Req) {
 // These should be returned by the caller.
 func (h *StreamTypeHandler[Payload, Req, Resp]) PutResponse(r Resp) {
 	if r != h.nilResp {
-		if rc, ok := any(r).(Recycler); ok {
-			// Recycled stream responses must not leak fields
-			// from a previous use into the next response.
-			rc.Recycle()
-		}
 		//nolint:staticcheck // SA6002 IT IS A GENERIC VALUE! (and always a pointer)
 		h.respPool.Put(r)
 	}