chore(tests): prefer single item assertions

[yordis]

• Keep single-item assertions aligned with current xUnit analyzer guidance.
• Reduce test-diagnostic noise around metrics and scavenge coverage.

[cursor]

PR Summary

Low Risk
Low risk: test-only changes that refactor assertions without altering production behavior. Main risk is minor—tests may become stricter about expecting exactly one measurement/tag in a few cases.

Overview
Updates multiple metrics/scavenge-related tests to prefer single-item assertions by replacing Assert.Collection (with one expected element) with Assert.Single and then asserting on the returned measurement/tag/scavenge point.

This reduces analyzer noise and slightly tightens expectations around collections that should contain exactly one item (e.g., metric tags/measurements and newly-created scavenge points).

^{Reviewed by Cursor Bugbot for commit 034fd74. Bugbot is set up for automated code reviews on this repo. Configure here.}

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: a062dd91-3b81-46d7-b15c-61c6f5b97214

📥 Commits

Reviewing files that changed from the base of the PR and between 92e4a69 and 034fd74.

📒 Files selected for processing (5)

• src/EventStore.Core.XUnit.Tests/Metrics/IncomingGrpcCallsMetricTests.cs
• src/EventStore.Core.XUnit.Tests/Metrics/PersistentSubscriptionMetricsTests.cs
• src/EventStore.Core.XUnit.Tests/Metrics/QueueBusyTrackerTests.cs
• src/EventStore.Core.XUnit.Tests/Scavenge/SubsequentScavengeTests.cs
• src/EventStore.Core.XUnit.Tests/TransactionLog/Checkpoint/CheckpointMetricTests.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/chore-xunit-single-assertions

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