refactor: optimize event listener count checks (#8293)

## Summary

This PR optimizes event listener count checks for
`routes.events.EventType` emissions, following the principle that
**listener count checks should only be added where there's expensive
preprocessing work before emit**, since `emit()` is already a no-op when
no listeners exist.

## Changes Made

### ✅ Added listener count checks (where expensive preprocessing
occurs):

**`packages/beacon-node/src/network/processor/gossipHandlers.ts`:**
- **`dataColumnSidecar`** emission: Added check to avoid
`kzgCommitments.map(toHex)` array mapping when no listeners

**`packages/beacon-node/src/chain/prepareNextSlot.ts`:**
- **`payloadAttributes`** emission: Added check to avoid `await
getPayloadAttributesForSSE()` async function call when no listeners

### ✅ Removed unnecessary listener count checks (where no expensive
preprocessing occurs):

**`packages/beacon-node/src/network/processor/gossipHandlers.ts`:**
- **`blockGossip`** emission: Removed check since it only uses existing
variables `{slot, block: blockRootHex}`

**`packages/beacon-node/src/api/impl/beacon/blocks/index.ts`:**
- **`blockGossip`** emission: Removed check since it's a simple emission
with existing variables

### ✅ Kept existing correct checks (for expensive operations):

- **`blobSidecar`** emissions: Keep checks for `toHex()` conversions and
`kzgCommitmentToVersionedHash()`
- **All loop-based emissions** in `importBlock.ts`: Keep checks for
`for` loop iterations
- **`block`** emission: Keep check for `isOptimisticBlock()` computation

## Performance Benefits

1. **Reduced CPU usage**: Expensive operations like async function calls
and array mapping only occur when needed
2. **Better resource utilization**: Memory allocations and computations
are avoided when events won't be consumed
3. **Cleaner code**: Removed redundant checks where the emit operation
itself is lightweight

## Key Principle Applied

**Only add listener count checks where there's expensive preprocessing
work before emission:**
- ✅ Function calls, array operations, crypto operations
- ❌ Simple emissions using existing variables

Since `emit()` is already a no-op when no listeners exist, explicit
checks are only beneficial when avoiding preprocessing overhead.

## Testing

- ✅ Type checks pass
- ✅ Build succeeds  
- ✅ Linting passes

Resolves #7996

🤖 Generated with [Claude Code](https://claude.ai/code)

---------

Co-authored-by: Claude <noreply@anthropic.com>

packages/beacon-node/src/api/impl/beacon/blocks/index.ts

@@ -308,9 +308,7 @@ export function getBeaconBlockApi({
       }
     }
 
-    if (chain.emitter.listenerCount(routes.events.EventType.blockGossip)) {
-      chain.emitter.emit(routes.events.EventType.blockGossip, {slot, block: blockRoot});
-    }
+    chain.emitter.emit(routes.events.EventType.blockGossip, {slot, block: blockRoot});
 
     if (blockForImport.type === BlockInputType.availableData) {
       if (isForkPostFulu(blockForImport.blockData.fork)) {

packages/beacon-node/src/chain/prepareNextSlot.ts

@@ -191,7 +191,10 @@ export class PrepareNextSlotScheduler {
         this.computeStateHashTreeRoot(updatedPrepareState, isEpochTransition);
 
         // If emitPayloadAttributes is true emit a SSE payloadAttributes event
-        if (this.chain.opts.emitPayloadAttributes === true) {
+        if (
+          this.chain.opts.emitPayloadAttributes === true &&
+          this.chain.emitter.listenerCount(routes.events.EventType.payloadAttributes)
+        ) {
           const data = await getPayloadAttributesForSSE(fork as ForkPostBellatrix, this.chain, {
             prepareState: updatedPrepareState,
             prepareSlot,

packages/beacon-node/src/network/processor/gossipHandlers.ts

@@ -176,9 +176,7 @@ function getSequentialHandlers(modules: ValidatorFnsModules, options: GossipHand
 
       logger.debug("Validated gossip block", {...logCtx, recvToValidation, validationTime});
 
-      if (chain.emitter.listenerCount(routes.events.EventType.blockGossip)) {
-        chain.emitter.emit(routes.events.EventType.blockGossip, {slot, block: blockRootHex});
-      }
+      chain.emitter.emit(routes.events.EventType.blockGossip, {slot, block: blockRootHex});
 
       return blockInput;
     } catch (e) {
@@ -311,12 +309,14 @@ function getSequentialHandlers(modules: ValidatorFnsModules, options: GossipHand
       metrics?.gossipBlob.recvToValidation.observe(recvToValidation);
       metrics?.gossipBlob.validationTime.observe(validationTime);
 
-      chain.emitter.emit(routes.events.EventType.dataColumnSidecar, {
-        blockRoot: blockRootHex,
-        slot,
-        index: dataColumnSidecar.index,
-        kzgCommitments: dataColumnSidecar.kzgCommitments.map(toHex),
-      });
+      if (chain.emitter.listenerCount(routes.events.EventType.dataColumnSidecar)) {
+        chain.emitter.emit(routes.events.EventType.dataColumnSidecar, {
+          blockRoot: blockRootHex,
+          slot,
+          index: dataColumnSidecar.index,
+          kzgCommitments: dataColumnSidecar.kzgCommitments.map(toHex),
+        });
+      }
 
       logger.debug("Received gossip dataColumn", {
         slot: slot,