test(cluster): tighten the test suite and fix crate docs

[iamquang95]

Hardens pluto-cluster's tests where they asserted less than they appeared to, and corrects docs that described pluto's own types as charon's.

• threshold pinned to a ceil(2n/3) oracle table (n=1..22) plus an exact-ceil property check
• Dropped the silent http://127.0.0.1:8545 dependency — 22 sites now use EthClient::Noop
• Example fixtures parse from their pristine bytes; the back-filling patcher is gone (no parser gap found)
• Version tests assert parsed fields instead of let _ = from_str::<V>()
• New coverage: ssz hasher error paths, verify_sig branches, agg_sign round-trip, every Definition::new validation error