fix: export UndiciHeaders type and set dispatch headers to UndiciHeaders #15699

Job	Run time
dependency-review	14s
Lint	43s
Fuzzing	3m 37s
Test with SQLite enabled	33s
Test TypeScript types	42s
Test with Node.js 20 compiled --without-intl	4m 34s
Test with Node.js 22 compiled --without-intl	4m 23s
Test with Node.js 23 compiled --without-intl	4m 12s
test (20, macos-latest) / Test with Node.js 20 on macos-latest	5m 5s
test (20, windows-latest) / Test with Node.js 20 on windows-latest	4m 55s
test (20, ubuntu-latest) / Test with Node.js 20 on ubuntu-latest	3m 44s
test (22, macos-latest) / Test with Node.js 22 on macos-latest	3m 10s
test (22, ubuntu-latest) / Test with Node.js 22 on ubuntu-latest	4m 13s
test (22, windows-latest) / Test with Node.js 22 on windows-latest	4m 35s
test (23, macos-latest) / Test with Node.js 23 on macos-latest	1m 43s
test (23, ubuntu-latest) / Test with Node.js 23 on ubuntu-latest	3m 24s
test (23, windows-latest) / Test with Node.js 23 on windows-latest	4m 38s
automerge	0s
	54m 25s

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