fix: subscribe read nodes to MEMPOOL_TOPIC

[manan19]

Summary

• Fixes Read nodes don't subscribe to MEMPOOL_TOPIC, breaking submit-to-read-node propagation #865.
• Read nodes never subscribed to MEMPOOL_TOPIC, so they couldn't join the mempool gossip mesh after chore: enable submit to read nodes #349 enabled submit_message on them. This made every client submission a publish over the topic-fanout fallback (best-effort, TTL'd, lossy under poor connectivity), and dropped all inbound mempool gossip wholesale.
• Hoist the MEMPOOL_TOPIC subscription out of the validator-only branch so both node types subscribe to it.
• Add a regression test (test_read_node_receives_mempool_gossip) that asserts a read node delivers mempool messages published by a validator peer to its system channel.

Test plan

• cargo check --tests passes
• cargo test --lib network::gossip_tests passes (3/3, including the new regression test)
• CI green

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Project	Deployment	Actions	Updated (UTC)
snapchain-docs	Ready	Preview, Comment	Apr 30, 2026 3:50pm

[Copilot]

Pull request overview

This PR fixes a network gossip regression where read nodes did not subscribe to MEMPOOL_TOPIC, preventing them from joining the mempool gossipsub mesh and receiving inbound mempool gossip (impacting submit-to-read-node propagation).

Changes:

• Subscribe all node types (validators and read nodes) to MEMPOOL_TOPIC during gossip initialization.
• Add a regression test ensuring a read node receives mempool gossip published by a validator peer.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/network/gossip.rs	Hoists MEMPOOL_TOPIC subscription out of the validator-only branch so read nodes also subscribe and receive mempool gossip.
src/network/gossip_tests.rs	Adds a regression test verifying a read node delivers mempool gossip to its system channel.

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[github-actions]

Diff Coverage

Diff: origin/main...HEAD, staged and unstaged changes

• src/network/gossip.rs (60.0%): Missing lines 328-329

Summary

• Total: 5 lines
• Missing: 2 lines
• Coverage: 60%

src/network/gossip.rs

  324         // back to fanout (best-effort, TTL'd, not in the mesh).
  325         let topic = gossipsub::IdentTopic::new(MEMPOOL_TOPIC);
  326         let result = swarm.behaviour_mut().gossipsub.subscribe(&topic);
  327         if let Err(e) = result {
! 328             warn!("Failed to subscribe to topic: {:?}", e);
! 329             return Err(Box::new(e));
  330         }
  331 
  332         let topic = gossipsub::IdentTopic::new(CONTACT_INFO);
  333         let result = swarm.behaviour_mut().gossipsub.subscribe(&topic);

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

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