exec-server: use virtual time in Noise relay test

[bolinfest]

Why

fragmented_writes_yield_to_keepalive_and_queued_pong deliberately blocks WebSocket writes while exercising keepalive and queued-Pong scheduling. It previously advanced those states with wall-clock sleeps. Under a sufficiently delayed CI worker, those sleeps and scheduling gaps could consume the test-only 100 ms Pong-watchdog budget, causing the relay to exit and the next write-permit send to fail with TrySendError::Disconnected.

The failure was therefore a timing flake in the harness test, not evidence that the production relay mishandled a Pong.

What changed

• Run this test with Tokio time paused.
• Advance the virtual clock through its two keepalive transitions instead of sleeping in wall-clock time.
• Enable Tokio's test-util feature only for codex-exec-server dev dependencies.

No production code or timeout values change.

Review guide

The behavioral change is confined to noise_relay/harness_tests.rs; the Cargo.toml change only exposes Tokio's paused-clock test APIs.

Validation

• just test -p codex-exec-server fragmented_writes_yield_to_keepalive_and_queued_pong
• just fix -p codex-exec-server
• just bazel-lock-update (no lockfile changes)