refactor(errors): finish thiserror migration and demote anyhow to dev-deps

[varex83agent]

Closes #612.

Replaced the hand-written Display/std::error::Error impls with #[derive(thiserror::Error)] for peerinfo::Failure, app::SharedStepError, p2p::PeerNotAllowed, and dkg::SenderPeerMismatch (Display strings preserved verbatim; #[source] added where a cause was previously exposed). Added #[source] to the five source-carrying K1UtilError variants and, from the workspace sweep, the five pluto_crypto::types::Error variants wrapping BlsError (identical pattern, in-scope crate). Replaced featureset's two TryFrom<&str> type Error = String impls with typed ParseStatusError/ParseFeatureError (same rendered messages) and switched beacon.rs to the typed CliError::TestCaseNotSupported (message text "test case not supported" unchanged, asserted-on tests still pass).

Skipped / already-done, noted here rather than guessed:

• validatorapi::ApiError was NOT converted: its Display is conditional (renders the source inline only when present), which thiserror 2.0 can only express via #[error(fmt = ...)] on enums — not structs. Keeping a hand-written Display + Error was the only way to preserve the exact output, so the manual impls remain. This is a public-facing message contract, not worth a lossy rewrite.
• anyhow demotion: consensus, p2p, parsigex, peerinfo, and dkg already declare anyhow only under [dev-dependencies] on main (verified src/ non-test has zero anyhow; dkg's uses are all in #[cfg(test)] modules), so no Cargo.toml changes were needed.
• From<BLST_ERROR> for Error / Error::BlsError: no longer present in crates/crypto/src/types.rs; only From<BLST_ERROR> for BlsError exists and BLST failures are already routed to typed Error variants at each call site, so there was nothing to re-route.
• Workspace sweep found the same {0}-without-#[source] pattern in ssz/cluster/eth2util/consensus; left untouched to keep the change scoped to the issue's crate list.

eth2api deliberately untouched (out of scope, #611).

Quality gates (affected crates): cargo +nightly fmt --all --check, cargo clippy ... -D warnings, and cargo test --all-features all pass.

Co-Authored-By: Bohdan Ohorodnii 35969035+varex83@users.noreply.github.com