Add copts.defaultRuntime to NewWithConn() when it is specify the ops.

[yanggangtony]

/kind cleanup
/kind bug

We should not omit the copts.defaultRuntime when it is actually passd in from the Outer of Method func NewWithConn(

Demo code:

import (
	containerd "github.com/containerd/containerd/v2/client"
) 
func main(){
xxx

containerd.NewWithConn(xxx)

xxx
}

If before conn has set the runtime , when use the containerd.NewWithConn() , will drop the before runtime set , and use default set .

[k8s-ci-robot]

Hi @yanggangtony. Thanks for your PR.

I'm waiting for a containerd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[yanggangtony]

Hey, friendly ping .
This pr is for a long time .
Would someone put it for progress?