chore(coderd/x/chatd/chatdebug): clean up after the chatd refactor (#26345)

Addresses
https://github.com/coder/coder/pull/26109#discussion_r3379164243 and
https://github.com/coder/coder/pull/26109#discussion_r3379151284

coderd/x/chatd/chatdebug/service.go

@@ -524,7 +524,9 @@ func (s *Service) TouchStep(
 
 // TouchRun bumps the run's updated_at timestamp without changing any
 // other fields. Runner-owned debug turns use this while no model step is
-// active, such as requires-action waits.
+// active, such as requires-action waits. It is a no-op when the service
+// is nil or either ID is uuid.Nil, since debug tracking is optional and
+// callers may not have an open run.
 func (s *Service) TouchRun(ctx context.Context, runID uuid.UUID, chatID uuid.UUID) error {
 	if s == nil || runID == uuid.Nil || chatID == uuid.Nil {
 		return nil
@@ -538,7 +540,9 @@ func (s *Service) TouchRun(ctx context.Context, runID uuid.UUID, chatID uuid.UUI
 }
 
 // LaunchRunHeartbeat starts a goroutine that periodically touches an
-// open run until done is closed or ctx is canceled.
+// open run until done is closed or ctx is canceled. It is a no-op when
+// the service is nil, either ID is uuid.Nil, or done is nil, since debug
+// tracking is optional and callers may not have an open run.
 func (s *Service) LaunchRunHeartbeat(ctx context.Context, runID uuid.UUID, chatID uuid.UUID, done <-chan struct{}) {
 	if s == nil || runID == uuid.Nil || chatID == uuid.Nil || done == nil {
 		return
@@ -548,7 +552,7 @@ func (s *Service) LaunchRunHeartbeat(ctx context.Context, runID uuid.UUID, chatI
 		interval := s.heartbeatInterval()
 		ticker := s.clock.NewTicker(interval, "chatdebug", "run-heartbeat")
 		defer ticker.Stop()
-		resetTicker := func() {
+		maybeResetTicker := func() {
 			if newInterval := s.heartbeatInterval(); newInterval != interval {
 				interval = newInterval
 				ticker.Reset(interval, "chatdebug", "run-heartbeat")
@@ -562,7 +566,7 @@ func (s *Service) LaunchRunHeartbeat(ctx context.Context, runID uuid.UUID, chatI
 				return
 			case <-thresholdCh:
 				thresholdCh = s.thresholdChan()
-				resetTicker()
+				maybeResetTicker()
 			case <-ticker.C:
 				if err := s.TouchRun(ctx, runID, chatID); err != nil {
 					s.log.Debug(ctx, "run heartbeat touch failed",
@@ -570,7 +574,7 @@ func (s *Service) LaunchRunHeartbeat(ctx context.Context, runID uuid.UUID, chatI
 						slog.F("run_id", runID),
 					)
 				}
-				resetTicker()
+				maybeResetTicker()
 			}
 		}
 	}()