XY-872: Stabilize Decodex full release gate for v0.2.0

[yvette-carlisle]

Summary

• Stabilizes the operator dashboard HTTP/WebSocket test harness by replacing duplicated 1s socket/event deadlines with one shared 5s test deadline.
• Keeps all assertions and coverage intact; no tests are skipped and no release-gate command semantics are changed.

Failing Tests Observed

• Current full-gate reproduction did not emit app-server timeout test names: after restoring checked-in site dependencies, cargo make check and cargo make test both completed.
• The first baseline run stopped before nextest because the local site dependency executable was missing (astro: command not found); npm ci in site/ restored the locked dependency tree and left no tracked-file changes.
• The timeout-sensitive app-server-adjacent surfaces exercised in this lane were the app_server, operator_dashboard_websocket, and operator_state_endpoint nextest filters.

Root Cause / Stabilization

• The likely remaining full-gate-only flake surface was the operator dashboard HTTP/WebSocket harness using 1s wall-clock read/event deadlines while the full release gate was compiling and running many tests concurrently.
• Focused reruns can pass because they avoid that load; the harness now uses a 5s bounded test deadline, preserving failure behavior for real hangs while reducing false timeouts under release-gate load.

Validation

• cargo nextest run -p decodex --all-targets --all-features operator_dashboard_websocket --stress-count 5 --no-fail-fast
• cargo nextest run -p decodex --all-targets --all-features operator_state_endpoint --stress-count 5 --no-fail-fast
• cargo nextest run -p decodex --all-targets --all-features app_server --stress-count 5 --no-fail-fast
• cargo make fmt
• cargo make lint-fix
• cargo make test
• cargo make check