test(request): reject duplicate singleton headers

[ronag]

Summary

• Port the four request-header boundary cases from nodejs/undici commit nodejs@74495c6.
• Cover same-case and case-variant duplicates of Host and Content-Length in flat header arrays.
• Assert validation rejects before any connector or network activity.

The lower runtime already has the upstream rejection logic, so this is deliberately a test-only port.

Validation

• npm run test:unit -- --no-typescript (579 passing, 0 failing, 6 skipped)
• npm run test:node-test -- --no-typescript (151 passing, 0 failing)
• npm run test:types
• npm run lint -- --no-cache
• node --test test/headers-as-array.js (14 passing)
• git diff --check

Copilot reviewed the exact head commit with no actionable findings.

[Copilot]

Pull request overview

This PR ports upstream Undici test cases to ensure Client.request() rejects duplicate singleton headers (Host and Content-Length) when headers are provided as a flat array, and verifies the rejection happens before any connection attempt.

Changes:

• Added a shared helper to assert duplicate-header validation rejects and that no connector/network activity occurs.
• Added four new tests covering same-case and case-variant duplicates of Host and Content-Length in flat header arrays.

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