fix(http_fetcher): use undici.fetch when dispatcher is present (#4097)

### 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`.

```js
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.

----

Fixes #4093

tests/unit/functions/http_fetcher_spec.js

@@ -440,3 +440,32 @@ describe("fetch() method", () => {
 		expect(errorInfo.errorType).toBe("NETWORK_ERROR");
 	});
 });
+
+describe("selfSignedCert dispatcher", () => {
+	const { Agent } = require("undici");
+
+	it("should set rejectUnauthorized=false when selfSignedCert is true", () => {
+		fetcher = new HTTPFetcher(TEST_URL, {
+			reloadInterval: 60000,
+			selfSignedCert: true
+		});
+
+		const options = fetcher.getRequestOptions();
+
+		expect(options.dispatcher).toBeInstanceOf(Agent);
+		const agentOptionsSymbol = Object.getOwnPropertySymbols(options.dispatcher).find((s) => s.description === "options");
+		const dispatcherOptions = options.dispatcher[agentOptionsSymbol];
+		expect(dispatcherOptions.connect.rejectUnauthorized).toBe(false);
+	});
+
+	it("should not set a dispatcher when selfSignedCert is false", () => {
+		fetcher = new HTTPFetcher(TEST_URL, {
+			reloadInterval: 60000,
+			selfSignedCert: false
+		});
+
+		const options = fetcher.getRequestOptions();
+
+		expect(options.dispatcher).toBeUndefined();
+	});
+});