feat(testutil/validatormock): port attest.go (SlotAttester)

Ports `charon/testutil/validatormock/attest.go` to Rust. SlotAttester drives
Prepare -> Attest -> Aggregate for a single slot, gated by Arc<CloseOnce>
close-once flags (AtomicBool + Notify) that mirror Go's `chan struct{}` ready
signals; mutable state lives behind Arc<Mutex<_>> so all entry points stay
`&self`.

Because the Rust eth2api client encodes attestations using the Electra
SingleAttestation shape (incompatible with Go's VersionedAttestation JSON
captured in the goldens), the two submit endpoints
(`POST /eth/v2/beacon/pool/attestations` and
 `POST /eth/v2/validator/aggregate_and_proofs`) are POSTed as raw JSON whose
structure matches `eth2spec.VersionedAttestation` /
`*SubmitAggregateAttestationsOpts`. All other beacon-node interactions use
the generated client.

TestAttest matches Go's golden output for DutyFactor {0, 1} on validator
set A via assert_json_eq, with submissions sorted by data.index to match
the Go test's deterministic ordering.

Also adds Eth2Exp and Submit variants to validatormock::Error, and a
ValidatorSet-driven POST `/eth/v1/beacon/states/head/validators` plus a
BeaconCommitteeSelections echo route in the tests because the beaconmock
defaults don't cover those DV-only paths.

Author: varex83

Cargo.lock

@@ -22,9 +22,12 @@ pluto-core.workspace = true
 pluto-crypto.workspace = true
 pluto-eth2api.workspace = true
 pluto-eth2util.workspace = true
+pluto-ssz.workspace = true
 rand.workspace = true
+reqwest.workspace = true
 serde.workspace = true
 serde_json.workspace = true
+serde_with.workspace = true
 thiserror.workspace = true
 tokio.workspace = true
 tokio-util.workspace = true
@@ -34,7 +37,6 @@ wiremock.workspace = true
 
 [dev-dependencies]
 assert-json-diff.workspace = true
-reqwest.workspace = true
 
 [lints]
 workspace = true