deps(nuget): Bump xunit.runner.visualstudio from 3.1.5 to 4.0.0

[dependabot]

Updated xunit.runner.visualstudio from 3.1.5 to 4.0.0.

Release notes

Sourced from xunit.runner.visualstudio's releases.

4.0.0

Release notes: https://xunit.net/releases/visualstudio/4.0.0

4.0.0-pre.5

Release notes: https://xunit.net/releases/visualstudio/4.0.0-pre.5

4.0.0-pre.4

Release notes: https://xunit.net/releases/visualstudio/4.0.0-pre.4

4.0.0-pre.3

Release notes: https://xunit.net/releases/visualstudio/4.0.0-pre.3

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[Chris0Jeky]

Coordinator technical review — exact head c0cb3c46b0db94b3ef3fbc930bc50291dbbf8381

Disposition: no direct CRITICAL/HIGH defect found.

Evidence:

• repository remains xUnit v2 on .NET 8; runner 4.0.0 retains VSTest support for xUnit v1/v2/v3 and no custom runner configuration or v3-only surface was found;
• local dotnet build backend/Taskdeck.sln -c Release -m:1 --no-restore passed with 0 errors and 13 existing compiler warnings (no package deprecation warning);
• local full solution tests passed: 7,813 passed, 5 intentionally skipped, 0 failed across Domain, Application, API, CLI, Architecture, and PostgreSQL integration projects;
• exact-head required CI passed the same core test projects on Ubuntu and Windows; current CI Extended checks are green. Scheduled nightly coverage was not part of this PR run.

This is a major-version update, so docs/ops/DEPENDENCY_UPDATE_POLICY.md requires explicit maintainer assessment. No merge was performed.

[Chris0Jeky]

ADR-0051 removes the former human-category merge gate. This PR is staged as Project Next / Priority V behind #1687 because both edit backend/Directory.Packages.props. After #1687 lands, refresh against live main, rerun Release build/full test discovery and exact-head CI/review, then merge.