[test][node] Cover join stage-1 with no known right neighbor

[TheP2P (thep2p)]

Summary

• Add test_join_stage1_link_level0_no_right_neighbor_sends_single_link_request: when s's neighbor query resolves z as None, join_stage1_link_level0 never sends a second GetLinkOp and resolves once the single s-side request is applied, leaving the right-side table entry unset.
• Required acceptance-criteria coverage for [Node] Implement: level-0 join linking (Phase 1) #90 (issue [Node] Implement: level-0 join linking (Phase 1) #90's third required test scenario). No production code change: the None-handling branch this exercises already shipped in the orchestration PR earlier in this stack.

Test plan

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