fix: delay debounce until the first event

[immanuwell]

ControlStream starts waiting on nsDebounce.Triggered() right away. Before this fix, Triggered() armed the timer on first read, so an idle stream could send an empty namespaces update after 100ms even when no namespace event ever happened. Kinda noisy.

Fix is small:

• keep debounce idle until the first Touch()
• add tests for no pre-touch trigger, normal trigger, and reset-on-touch

Repro:

1. On master, run this in backend:

cat >/tmp/repro.go <<'EOF'
package main

import (
  "fmt"
  "time"

  "github.com/cilium/hubble-ui/backend/pkg/debounce"
)

func main() {
  d := debounce.New(50 * time.Millisecond)
  select {
  case <-d.Triggered():
    fmt.Println("triggered without touch")
  case <-time.After(120 * time.Millisecond):
    fmt.Println("no trigger")
  }
}
EOF
go run /tmp/repro.go

2. On master it prints triggered without touch.
3. With this branch it prints no trigger.
4. go test ./... still passes.

[immanuwell]

@geakstr please take a look