poll: do not cancel ephemeral GC until Connect succeeds

With node.ephemeral.inactivity_timeout set, ephemeral nodes are
usually deleted after they go offline, but under reconnect churn some
departed nodes stayed in the node list as disconnected indefinitely
until removed manually or until Headscale restarted.

Ephemeral cleanup is timer-based via EphemeralGarbageCollector, not a
periodic LastSeen scan. serveLongPoll cancelled any pending GC timer
at the very start of a long-poll attempt and only rescheduled on a
clean disconnect after Connect. If a reconnect cancelled the timer and
then failed before Connect (for example an UpdateNodeFromMapRequest
error), the deferred cleanup saw connectGen == 0 and returned without
Schedule. The node remained offline with no deletion timer and no
reconciler to recover it.

Cancel the ephemeral GC timer only after a successful Connect, so a
failed reconnect leaves an already-armed inactivity timer intact.
Successful reconnects still cancel GC once the node is online, and a
later disconnect reschedules as before.

Add TestFailedReconnectDoesNotCancelEphemeralGC to lock in the
ordering, plus IsScheduled and DeleteNodeFromStoreForTest helpers for
the test.

Fixes #3382

Co-authored-by: Cursor <cursoragent@cursor.com>

hscontrol/db/node.go

@@ -498,6 +498,16 @@ func (e *EphemeralGarbageCollector) Cancel(nodeID types.NodeID) {
 	}
 }
 
+// IsScheduled reports whether a deletion timer is currently armed for nodeID.
+func (e *EphemeralGarbageCollector) IsScheduled(nodeID types.NodeID) bool {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	_, ok := e.toBeDeleted[nodeID]
+
+	return ok
+}
+
 // Start starts the garbage collector.
 func (e *EphemeralGarbageCollector) Start() {
 	for {