Add internal/pause coverage tests

[hayabusa-cloud]

Summary

This PR raises test coverage for spin by adding focused unit tests for the internal CPU pause primitive.

What changed

• Added unit tests for internal/pause:
  • internal/pause/impl_test.go
• Covered the control-flow branches in Pause(cycles) (single-minimal pause vs pauseN).
• Added an allocation check to keep the hot path allocation-free (testing.AllocsPerRun).

Notes

• Tests are deliberately small and do not mock core behavior.
• The allocation assertion is intended to protect performance-sensitive call sites.

Checklist

• go test -race ./... passes

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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