test: cover filescan parallel helper

[vku2018]

Summary

• adds dedicated tests for internal/filescan.Parallel
• covers input-order preservation under out-of-order goroutine completion
• covers nil/empty input, rejected items, single-item input, and 100-item input
• checks the observed concurrent callback count stays within the helper's concurrency limit

Validation

• git diff --check
• Could not run go test ./internal/filescan or go test -race ./internal/filescan locally because this Windows environment does not have go/gofmt installed (go: The term 'go' is not recognized...).

Closes #96