fix(raft): fix all clippy warnings

[slow2342]

Summary

• Remove unused import ConfChangeType in node.rs
• Fix unused variables (bootstrapped, messages, context)
• Replace Default::default() + field assignment with struct initializer
• Use !is_empty() instead of len() >= 1
• Remove useless vec! where array suffices

Changes

• src/raft/node.rs — Remove unused import, fix unused mut and variables
• src/raft/raftrs_store.rs — Use struct initializers, fix useless conversions
• src/cluster/discovery.rs — Use array instead of vec! for test data

How to verify

cargo clippy --all-targets -- -D warnings
cargo test --all-targets
cargo fmt -- --check