chore(deps): update dependency nsubstitute to v6

[thomhurst]

This PR contains the following updates:

Package	Type	Update	Change
NSubstitute (source)	nuget	major	5.3.0 → 6.0.0

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Release Notes

nsubstitute/NSubstitute (NSubstitute)

v6.0.0

No changes from Release Candidate 1.

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Configuration

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This PR has been generated by Mend Renovate.

[codacy-production]

Up to standards ✅

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[claude]

Code Review — PR #6394 (renovate/nsubstitute-6.x)

Change: Single-line dependency bump in Directory.Packages.props — NSubstitute 5.3.0 → 6.0.0 (major version).

Verification performed

• Checked out the PR branch and built the two projects that reference NSubstitute (TUnit.UnitTests, TUnit.Mocks.Benchmarks) against net8.0/net9.0/net10.0 — all builds succeeded with 0 errors (only pre-existing, unrelated warnings).
• Searched the repo for actual NSubstitute/Substitute. API usage — found none in test source under TUnit.UnitTests (the package is referenced but not directly used there), and usage in TUnit.Mocks.Benchmarks compiles cleanly against v6's API surface.
• No other files besides Directory.Packages.props are touched, so this is a self-contained, low-risk change with no source-generator or public-API surface impact — outside the scope of the dual-mode/snapshot-testing rules in this repo's CLAUDE.md.

Assessment

This is a mechanical Renovate-generated dependency bump with no code changes required to accommodate the major version. Given the clean build across all target frameworks and the absence of direct API usage that would be sensitive to NSubstitute's v6 breaking changes, I don't see any issues — no architectural or design concerns apply here since there's no code to review beyond the version pin.

No blocking issues found. Safe to merge once CI is green.