fix(gossipsub): prune empty topic entries from topics map

[lodekeeper]

Summary

Monorepo port of ChainSafe/js-libp2p-gossipsub#543.

This carries over the empty-topic pruning fix for the internal topics: Map<TopicStr, Set<PeerIdStr>> bookkeeping:

• prune empty topics entries on remote unsubscribe
• prune empty topics entries on peer disconnect
• avoid creating empty topics entries from unsubscribe-only updates
• add regression coverage for the above cases

Original standalone PR / discussion: ChainSafe/js-libp2p-gossipsub#543
Original issue context: ChainSafe/js-libp2p-gossipsub#542

Why reopen here?

wemeetagain asked for this to be reopened against the js-libp2p monorepo because js-libp2p-gossipsub will soon be archived.

Notes

This is intentionally scoped to topics only. I did not fold in any mesh cleanup changes here because mesh tracks local joined-topic state and deleting empty mesh topics on disconnect would interfere with heartbeat maintenance/regrafting.

Testing

• npm exec --workspace packages/gossipsub -- aegir lint
• focused gossip.spec.ts run currently trips pre-existing monorepo package type/build issues in unrelated test/utils/* files; this patch itself is limited to packages/gossipsub/src/gossipsub.ts and packages/gossipsub/test/gossip.spec.ts

AI disclosure

Initial heap analysis and patch drafting were assisted by AI tooling, then validated with local source inspection and review.