fix: remove incorrect transport close on DoH3 error

Remove the transport Close() call from DoH3 error handling path.
The transport is shared and reused across requests, and closing it
on error would break subsequent requests. The transport lifecycle
is already properly managed by the http.Client and the finalizer
set in newDOH3Transport().

doh.go

@@ -122,11 +122,6 @@ func (r *dohResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, erro
 	}
 	if err != nil {
 		err = wrapUrlError(err)
-		if r.isDoH3 {
-			if closer, ok := c.Transport.(io.Closer); ok {
-				closer.Close()
-			}
-		}
 		return nil, fmt.Errorf("could not perform request: %w", err)
 	}
 	defer resp.Body.Close()