fix(http_fetcher): use undici.fetch when dispatcher is present

[KristjanESPERANTO]

What's the problem?

The selfSignedCert option passes an undici Agent as dispatcher to fetch(). But Node's built-in fetch() and undici@8's Agent use different internal handler APIs - passing them together throws:

invalid onRequestStart method

What's the fix?

When selfSignedCert is enabled (i.e. a dispatcher is set), use undici's own fetch() instead of the global one. For all other requests, keep using globalThis.fetch.

const fetchFn = requestOptions.dispatcher ? undiciFetch : globalThis.fetch;

Why not just always use undici's fetch?

That would fix the crash - but it would break some tests. MSW (Mock Service Worker), which is used in our test suite to intercept HTTP requests, only hooks into globalThis.fetch. Undici's fetch bypasses those interceptors entirely, so tests would start making real network requests instead of getting the mocked responses. We could rewrite all tests to use undici-compatible mocking instead - but that would be a massive change for no real benefit.

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Fixes #4093