test: verify PR output remains immutable

[k11kirky]

Problem

The PR output utility should remain a pure transformation so callers can safely reuse their existing output objects.

Changes

Add a regression test confirming buildPrOutput does not mutate its input.

How did you test this?

• git diff --check
• Automated test not run: repository dependencies are not installed, so biome and vitest are unavailable.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 682170e.}

[posthog]

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