fix(floodsub): apply decode limits and clean up the topic map

[tabcat]

Description

@libp2p/floodsub now bounds how many elements a single inbound RPC frame decodes into, and tidies up some peer/topic bookkeeping.

• Decode limits. Incoming RPCs are decoded with configurable element caps via a new decodeRpcLimits option (defaulting to 5000 subscriptions and 5000 messages per frame), so the number of elements decoded from one frame is bounded. This uses @libp2p/utils pbStream's decode-options support.
• Drop the unused control block. floodsub's RPC proto still carried gossipsub's ControlMessage (field 3), which floodsub never reads. It is removed (the field is reserved), so an inbound control block is skipped as an unknown field rather than decoded and discarded.
• Topic map bookkeeping. Empty peer sets are removed from the topic map on unsubscribe and peer removal, and the map is cleared on stop(), matching @libp2p/gossipsub.
• Peer teardown on inbound error. When a peer's inbound stream errors, the peer is now closed and removed instead of being left with a dead inbound stream.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works