Bug(p2p): --p2p-relays rejects URLs with a path

[iamquang95]

Description

Pluto rejects relay URLs containing a path; Charon accepts them. Relay addresses round-trip through Multiaddr, which cannot represent a path. This is a type-level limitation, not just a parsing bug.

Reproduction

# Charon v1.7.1 — accepted ("p2p-relays": "[http://relay:3640/enr]")
docker run --rm obolnetwork/charon:v1.7.1 run --p2p-relays=http://relay:3640/enr --simnet-beacon-mock

# Pluto — rejected at parse time
pluto run --p2p-relays=http://relay:3640/enr --simnet-beacon-mock
# → Error: Invalid multiaddr: invalid multiaddr

Acceptance criteria

• Both http://relay:3640/enr and http://relay:3640 work.
• Coverage for both URL forms, https://, and raw multiaddr.