Upgrade to libp2p 0.56.0

[Dentosal]

Bump libp2p to latest version. Vendors libp2p-swarm 0.45.1 to avoid a version conflict for version-compatibility tests: libp2p-swarm 0.45 (required by fuel-core-bin 0.44.0) pins libp2p-swarm-derive=0.35.0, while libp2p-swarm 0.47 (required by new libp2p 0.56) pins =0.35.1. They share the same 0.35.x semver bucket, so Cargo can't handle that.

Checklist

• Breaking changes are clearly marked as such in the PR description and changelog
• New behavior is reflected in tests
• The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• I have reviewed the code myself
• I have created follow-up issues caused by this PR and linked them here

[cursor]

PR Summary

Medium Risk
Medium risk due to a major libp2p dependency upgrade impacting networking behaviours, request/response events, and metrics wiring. Most code changes are mechanical API adaptations, but runtime connectivity and scoring/metrics should be re-validated.

Overview
Upgrades P2P dependencies, notably libp2p to 0.56.0 and prometheus-client to 0.23.0, with corresponding lockfile churn and an added changelog entry.

Updates the P2P service to compile against new libp2p APIs: gossipsub metrics are now attached via .with_metrics(...), request/response events handle new struct fields, Swarm::new_ephemeral_tokio is used in tests, and PublishError::InsufficientPeers handling is switched to NoPeersSubscribedToTopic.

Cleans up removed/obsolete deps and types (e.g., replaces void::Void with Infallible and void::unreachable with exhaustive matches), adds an explicit libp2p-gossipsub dependency to satisfy unused-dep linting, and introduces a version-compatibility workspace patch that vendors libp2p-swarm 0.45.1 to resolve a libp2p-swarm-derive version conflict.

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