fix: stop SlotIntervalTicker goroutine leaks (#16241)

`SlotIntervalTicker` was created in `prepareForkChoiceAtts` and in the
fork-choice attestation processing routine without ever calling `Done()`
when the service context was cancelled. Once the consuming goroutine
exits on `ctx.Done()`, the ticker keeps running and eventually blocks on
sending to its channel, leaving a leaked goroutine behind.

This change wires the lifetime of the `SlotIntervalTicker` to the
corresponding service contexts by calling `ticker.Done()` on
`ctx.Done()` in both call sites. This keeps the behaviour of the
routines unchanged while ensuring the ticker goroutines exit cleanly on
shutdown, consistent with how regular `SlotTicker` is handled elsewhere
in the codebase.

beacon-chain/blockchain/receive_attestation.go

@@ -94,6 +94,7 @@ func (s *Service) spawnProcessAttestationsRoutine() {
 		for {
 			select {
 			case <-s.ctx.Done():
+				ticker.Done()
 				return
 			case slotInterval := <-ticker.C():
 				if slotInterval.Interval > 0 {

beacon-chain/operations/attestations/prepare_forkchoice.go

@@ -49,6 +49,7 @@ func (s *Service) prepareForkChoiceAtts() {
 			}
 		case <-s.ctx.Done():
 			log.Debug("Context closed, exiting routine")
+			ticker.Done()
 			return
 		}
 	}

changelog/galoretka_fix-leak.md

@@ -0,0 +1,3 @@
+### Fixed
+
+- stop SlotIntervalTicker goroutine leaks [#16241](https://github.com/OffchainLabs/prysm/pull/16241)