fix(omp): honor execution turn duration

[cooper (czxtm)]

Summary

• forward validated execution max_duration_ms through trusted harness metadata
• use the execution duration plus a five-second grace for the OMP RPC safety deadline
• retain the historical 300-second fallback when metadata is absent or invalid

Verification

• cargo test --manifest-path crates/harness-server/Cargo.toml (133 passed, 5 ignored)
• cargo test --manifest-path services/api-rs/Cargo.toml -p centaur-session-runtime (139 passed)
• built centaur-api-rs:latest and centaur-agent:latest from the rebased commit
• rustfmt --edition 2024 --check on both changed files