Treat pending land checks as wait

[yvette-carlisle]

Summary

• classify pending/expected land checks as an explicit wait condition before generic mergeStateStatus rejection
• document that pending checks must keep retained review state and active ownership until CI is green

Validation

• cargo test -p decodex landing_state_validation -- --nocapture
• cargo test -p decodex pull_request::tests::landing_gates_handle_green_pending_and_review_request_cases -- --nocapture
• cargo make check