feat(testutil/validatormock): port propose.go (block proposal + builder reg)

Ports `propose_block` and `register` from
`charon/testutil/validatormock/propose.go`. Versioned dispatch matches the Go
switch on `eth2spec.DataVersion`; blinded path is gated on the proposal's
`execution_payload_blinded` flag (the Pluto generated client's analog of Go's
`block.Blinded`).

Phase0/Altair branches return `Error::UnsupportedVariant` until the Pluto
typed surface for them is ready; Bellatrix .. Fulu (full + blinded) are
implemented in full.

`register` ports the *intended* behaviour of Go's `Register`: it switches on
the input registration's `Version` (Go switches on the zero-value-initialized
signed registration version, which always falls through to V1 — a Go quirk
noted inline).

Tests mirror `TestProposeBlock` and `TestProposeBlindedBlock`; variants whose
random fixtures don't yet exist in `pluto-testutil::random` are `#[ignore]`d
with a TODO pointing at the missing helper.

Author: varex83

crates/testutil/src/validatormock/mod.rs

@@ -8,11 +8,13 @@
 pub mod capture;
 pub mod error;
 pub mod meta;
+pub mod propose;
 pub mod sign;
 pub mod validators;
 
 pub use capture::{EndpointMatch, SubmissionCapture};
 pub use error::{Error, Result, SignError};
 pub use meta::{MetaEpoch, MetaSlot, SpecMeta};
+pub use propose::{VersionedValidatorRegistration, propose_block, register};
 pub use sign::{Sign, SignFunc, Signer};
 pub use validators::{ActiveValidators, active_validators};