test: scope expired proposal diagnostics

[Chris0Jeky]

Summary

• Scope the GetExpiredAsync boundary test's assertions to its own corr-boundary-* correlation rows.
• Keep the production repository query and the worker-disabled integration fixture unchanged.
• This limits failure output from unrelated rows in the shared per-class database without weakening the expired/unexpired positive and negative controls.

Verification

• Baseline targeted test before the change: 1 passed.
• dotnet test backend/tests/Taskdeck.Api.Tests/Taskdeck.Api.Tests.csproj --filter FullyQualifiedName~AutomationProposalRepositoryIntegrationTests.GetExpiredAsync_WithExpiresAtInTheFuture_ShouldNotReturnAsExpired --no-restore — 1 passed after the change.
• dotnet test backend/tests/Taskdeck.Api.Tests/Taskdeck.Api.Tests.csproj --filter FullyQualifiedName~AutomationProposalRepositoryIntegrationTests --no-restore — 30 passed.
• git diff --check — passed.

Two coordinator review passes found no correctness, scope, or test-hygiene blockers. The hosted automatic Codex review was unavailable because the account had reached its code-review usage limit; no merge is being requested here.

The test project emitted existing nullable warnings in unrelated test helpers; no new warning was introduced by this one-file test change.

Refs #1853

[chatgpt-codex-connector]

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