fix(p2p): send the client version as the identify agent version

[MysticRyuujin]

🗒️ Description / Motivation

The libp2p identify reply does not contain the client name. build_swarm does not call identify::Config::with_agent_version, so rust-libp2p sends its default agent version: rust-libp2p/0.48.0.

Network crawlers read the identify agentVersion to identify the client of a consensus-layer peer. Because of this, ethlambda nodes show as rust-libp2p, and crawlers cannot tell them apart from other rust-libp2p peers.

What Changed

• crates/net/p2p/src/lib.rs: add SwarmConfig::agent_version and give it to the identify config with with_agent_version.
• bin/ethlambda/src/main.rs: set agent_version to version::CLIENT_VERSION. This is the same string as /eth/v1/node/version and engine_getClientVersionV1, for example ethlambda/v0.1.0-<branch>-<sha>/<target>/rustc-v<version>.
• Test SwarmConfig literals: set agent_version: "ethlambda/test".
• New test identify_reports_the_configured_agent_version: two lean swarms connect, and the dialer reads the agent version from the identify reply of the listener.

Correctness / Behavior Guarantees

• The identify protocolVersion does not change (eth2/1.0.0 on beacon, /ipfs/0.1.0 on lean).
• The only change on the wire is the agentVersion string. It applies to the lean and the beacon networks.

Tests Added / Run

• identify_reports_the_configured_agent_version. Without the fix, it fails with left: "rust-libp2p/0.48.0".
• cargo test --locked -p ethlambda-p2p --lib: 209 passed, 1 ignored.
• cargo clippy --locked -p ethlambda-p2p -p ethlambda --all-targets -- -D warnings: clean.
• cargo fmt --all --check: clean.

Related Issues / PRs

• None.

✅ Verification Checklist

• Ran make fmt — clean
• Ran make lint (clippy with -D warnings) — clean (ran clippy on the two changed crates, not the full workspace)
• Ran make test (test-consensus plus test-node, at release-fast) — all passing (ran the ethlambda-p2p unit tests only)

[MegaRedHand]

Thanks for the PR!