Make extraction gate concurrency tests awaitable

[Chris0Jeky]

Fix extraction-gate test starvation under Windows CI by making saturation and cancellation coordination awaitable. TaskCompletionSource signals use asynchronous continuations so admitted extractor calls do not block pool threads, while preserving the cap, rejection, permit-return and cancellation assertions.

Validation at head 5e3e230ab2f250772ec33e3630c325b549ad8dbb, base f0eb7a336:

• Focused ArtefactExtractionGateTests: five consecutive runs, six passing tests each.
• dotnet test backend/Taskdeck.sln -c Release -m:1 --disable-build-servers: 9,559 passed, 34 existing skips, zero failures. Domain 1,676; Application 4,428; API 3,177 / 4 skipped; CLI 243; Architecture 28 / 1 skipped; Integration 7 / 29 skipped.
• Fresh-context independent review: clean. Existing unrelated compiler warnings and environment-gated skips remain.

Only the extraction test file changes. No product behavior or canonical-state change, so no docs update is needed. Human actions remain in OUTSTANDING_TASKS.md.

Closes #2993.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-12T14:39:43.908615Z	5e3e230	PR opened

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[Chris0Jeky]

Delivery is coordinated through integration PR #3050, which preserves this PR's exact source head and commits. It qualifies all reviewed product slices against main1931da65d after the Smart CI repair; no source merge-resolution changes were required. Local source evidence remains scoped to its recorded head, and the integration's hosted checks and bounded interaction review qualify the combined delivery. Existing review feedback is triaged once; this source PR will be reconciled after the integration lands.