Backport TestResubscribeWithCompletedSubscription and deadlock fix to release/3.3 (#18862)

Backports test case and deadlock fix from PR #18562 (commit
`f3e42e7626`) that validates `Unsubscribe()` doesn't deadlock when the
subscription producer exits before unsubscription.

## Changes

- **Test case**: `TestResubscribeWithCompletedSubscription` in
`p2p/event/subscription_test.go`
- Validates no deadlock when producer completes before `Unsubscribe()`
is called
  
- **Deadlock fix**: Make `resubscribeSub.unsub` channel buffered (size
1)
- Without buffer, `Unsubscribe()` blocks when sending after resubscribe
loop exits
  - Buffer allows send to complete even when no receiver is present

```go
// Before: unbuffered channel causes deadlock
unsub: make(chan struct{})

// After: buffered channel prevents deadlock
unsub: make(chan struct{}, 1)
```

The test reproduces the deadlock scenario by ensuring the subscription
completes successfully before calling `Unsubscribe()`.

## Original Source

- **Original PR**: #18562 - "Sync p2p/event with geth's event"
- **Original Commit**: f3e42e7

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<details>

<summary>Original prompt</summary>

> Backport the test case `TestResubscribeWithCompletedSubscription` from
the `main` branch of Erigon to the branch `release/3.3`. This test
validates that the `Unsubscribe` method does not result in a deadlock
when the producer exits first. Reference implementation in
`p2p/event/subscription_test.go` from commit
`b8d1242103d81d46591cf86c71d8e182bc8c2add`. Ensure it only modifies the
`release/3.3` branch and includes the additional test case.


</details>



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Author: Copilot

p2p/event/subscription.go

@@ -123,7 +123,7 @@ func ResubscribeErr(backoffMax time.Duration, fn ResubscribeErrFunc) Subscriptio
 		backoffMax: backoffMax,
 		fn:         fn,
 		err:        make(chan error),
-		unsub:      make(chan struct{}),
+		unsub:      make(chan struct{}, 1),
 	}
 	go s.loop()
 	return s

p2p/event/subscription_test.go

@@ -156,3 +156,27 @@ func TestResubscribeWithErrorHandler(t *testing.T) {
 		t.Fatalf("unexpected subscription errors %v, want %v", subErrs, expectedSubErrs)
 	}
 }
+
+func TestResubscribeWithCompletedSubscription(t *testing.T) {
+	t.Parallel()
+
+	quitProducerAck := make(chan struct{})
+	quitProducer := make(chan struct{})
+
+	sub := ResubscribeErr(100*time.Millisecond, func(ctx context.Context, lastErr error) (Subscription, error) {
+		return NewSubscription(func(unsubscribed <-chan struct{}) error {
+			select {
+			case <-quitProducer:
+				quitProducerAck <- struct{}{}
+				return nil
+			case <-unsubscribed:
+				return nil
+			}
+		}), nil
+	})
+
+	// Ensure producer has started and exited before Unsubscribe
+	close(quitProducer)
+	<-quitProducerAck
+	sub.Unsubscribe()
+}