test(relay): make mesh demo test use production's directory lease - #7402

Problem

mesh_demo::demo_join_forwarded_arm_round_trips_echo fails with a 504 under load and passes in isolation — a timing race, not a logic bug.

The test helper builds its SessionDirectory with a test-only 5-second lease override, while run_demo_join's echo timeout is 10 seconds and the demo join intentionally does not renew. The lease therefore expires mid-wait and the echo can never arrive in time. The detached owner task hides the fence expiry, so it surfaces as an opaque 504.

Adding tracing changed the scheduling enough for the same test to pass in 0.01s, which confirms the race.

Fix

Drop the test-only override and construct the directory exactly as production does:

mesh_boot.rs:720 SessionDirectory::new(pool) <- production
mesh_demo.rs:176 SessionDirectory::new(pool) <- now the test

SessionDirectory::new delegates to with_lease_ttl(pool, DEFAULT_LEASE_TTL) where DEFAULT_LEASE_TTL = 30s (tunnel/directory.rs:15,191).

Note this deliberately makes the test faithful rather than merely tolerant — inflating the TTL to some larger number would have widened the race window instead of removing the premise. The test was exercising a lease configuration production never uses.

Verification

Isolated test passes, exit 0. No production code changed — the diff is confined to #[cfg(test)] mod tests.

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