refactor(eth2api): make ValidatorCache immutable in BeaconNodeClient

[varex83agent]

Summary

BeaconNodeClient held its ValidatorCache as Arc<RwLock<Option<ValidatorCache>>>, initialized to None and populated later via set_validator_cache, so every read had to handle a "not yet set" case that leaked into callers (the scheduler's .expect("validator cache is available")).

The validator set is actually known at node construction (from the cluster's validators), so this optionality was never needed. This PR builds a single ValidatorCache once in node::run and threads it into both the scheduler and submission BeaconNodeClients at construction.

Changes

• BeaconNodeClient::new now takes the ValidatorCache and stores it as a plain field (ValidatorCache is already Arc-backed, so clones seeded into each client share state — a single per-epoch refresh still updates every consumer).
• Removed the Arc<RwLock<Option<..>>> wrapping, the set_validator_cache setter, and the NoActiveValidatorCache error variant.
• validator_cache() returns &ValidatorCache; dropped the .expect/TODO at the scheduler read site.
• Updated wiring (node::run / wire_core_workflow) to construct-and-seed the shared cache up front instead of set_validator_cache after the fact, and all tests to the new constructor.

Testing

• cargo +nightly fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings — clean
• cargo test -p pluto-eth2api -p pluto-core -p pluto-app — all pass (eth2api 133, core 605, app 141)

Closes #482

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