test(clusterloader2): add unit tests for pkg/errors

[HarnageaGabriel]

What this PR does

Adds unit test coverage for clusterloader2/pkg/errors, which had zero test files. Part of the broader effort tracked in #572 ("clusterloader2 should be covered with unit tests").

Covers all three source files in the package:

• error_list.go — ErrorList: IsEmpty, Append, Concat (including nil-argument case), Has (including errors.Is-wrapped error matching), String, Error
• fatal_error.go — NewErrCritical, verifying errors.Is unwraps to both ErrCritical and the original wrapped error
• metric_violation_error.go — NewMetricViolationError / IsMetricViolationError

Package coverage: 100%.

Relation to #3872

This does not overlap with #3872, which adds a single test (TestConvertSampleToHistogram) for pkg/measurement/util/histogram.go. This PR targets a different, currently-untested package (pkg/errors) chosen specifically to avoid duplicating that work.

Testing

go build ./...
go test ./pkg/errors/... -v -cover
gofmt -l pkg/errors/*_test.go
go vet ./pkg/errors/...

All pass, no other files modified.

[kubernetes-prow]

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[kubernetes-prow]

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: HarnageaGabriel
Once this PR has been reviewed and has the lgtm label, please assign wojtek-t for approval. For more information see the Code Review Process.

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Details

Needs approval from an approver in each of these files:

• clusterloader2/OWNERS

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