fix(gateway): harden scale-to-zero dormancy guards

[benbarclay]

Summary

• Block scale-to-zero dormancy while delegate_task(background=true) async delegations are still running.
• Restore gateway runtime status from draining back to running on the first real inbound after a dormant wake.
• Add regression tests for both review findings from PR feat(gateway): scale-to-zero idle detection + dormant-quiesce (Phase 0) #52207.

Test Plan

• uv run --extra dev python -m pytest tests/gateway/test_scale_to_zero.py tests/gateway/test_scale_to_zero_watcher.py tests/gateway/relay/test_relay_going_idle.py -q -o 'addopts='
• python -m compileall -q gateway/run.py tests/gateway/test_scale_to_zero_watcher.py
• git diff --check origin/main -- gateway/run.py tests/gateway/test_scale_to_zero_watcher.py

[github-actions]

🔎 Lint report: fix/scale-to-zero-review-warnings vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 11268 on HEAD, 11268 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 5942 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.

[tonydwb]

Code Review Summary

Verdict: Approved

This PR hardens the scale-to-zero dormancy guards in the gateway. Two key changes:

1. active_count() from tools.async_delegation is checked to prevent suspending while background delegations are active
2. _scale_to_zero_note_real_inbound() stamps real inbound traffic and restores lifecycle status after a dormant wake

Key observations

• Async delegation check prevents suspend while background tasks run
• Real inbound note prevents status from being stuck in draining after wake
• Internal completion/replay events intentionally do NOT call the helper
• Good test coverage for both changes

Looks Good

• Fail-closed design with defensive exception handling
• Proper separation between real traffic and internal events
• Tests verify the status restoration behavior

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Reviewed by Hermes Agent (cron)