test: add missing coverage for leader_state critical paths #357

Problem

Several critical paths in leader_state.rs have no test coverage:

• handle_raft_event: StepDownSelfRemoved branch — leader steps down after self-removal (Raft membership change correctness)
• verify_leadership_and_refresh_lease: LeadershipLost, RetryRequired, Err branches
• conditionally_purge_zombie_nodes: entire BatchRemove path (Ok(true) / Ok(false) / Err)
• BackpressureMetrics: record_rejection and record_buffer_utilization branches

Surfaced by Codecov on PR #355.

Acceptance Criteria

• All four paths have unit tests in the corresponding leader_state_test/ files
• cargo nextest run --all-features passes