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

js/http_fetcher.js

@@ -1,5 +1,5 @@
 const { EventEmitter } = require("node:events");
-const { Agent } = require("undici");
+const { fetch: undiciFetch, Agent } = require("undici");
 const Log = require("logger");
 const { getUserAgent } = require("#server_functions");
 
@@ -263,8 +263,13 @@ class HTTPFetcher extends EventEmitter {
 		const timeoutId = setTimeout(() => controller.abort(), this.timeout);
 
 		try {
-			const response = await fetch(this.url, {
-				...this.getRequestOptions(),
+			const requestOptions = this.getRequestOptions();
+			// Use undici.fetch when a custom dispatcher is present (e.g. selfSignedCert),
+			// because Node's global fetch and npm undici@8 Agents are incompatible.
+			// For regular requests, use globalThis.fetch so MSW and other interceptors work.
+			const fetchFn = requestOptions.dispatcher ? undiciFetch : globalThis.fetch;
+			const response = await fetchFn(this.url, {
+				...requestOptions,
 				signal: controller.signal
 			});
 

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();
+	});
+});