test: add Headers coverage and benchmark

[anonrig]

Node-side tests and benchmark only. No deps/undici changes.

• Implementation: perf: improve Headers class performance nodejs/undici#5699
• Node PR: test: add Headers coverage and benchmark nodejs/node#65365

  

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Incomplete benchmark DCE prevention

Medium Severity

The anti-DCE guard only observes result, but construct-*, set, append, and delete never assign it, so those timed loops can be eliminated. Other Node benchmarks keep the last constructed or mutated value alive with an assignment plus assert.ok. As written, those methods may report misleading throughput.

 

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