fix(vitest-pool-workers): enable dependency pre-bundling (#7810)

Author: edmundhung

.changeset/gold-frogs-rest.md

@@ -0,0 +1,29 @@
+---
+"@cloudflare/vitest-pool-workers": patch
+---
+
+Added [Vite dependency pre-bundling](https://vite.dev/guide/dep-pre-bundling) support. If you encounter module resolution issues—such as: `Error: Cannot use require() to import an ES Module` or `Error: No such module`—you can now bundle these dependencies using the [deps.optimizer](https://vitest.dev/config/#deps-optimizer) option:
+
+```tsx
+import { defineWorkersConfig } from "@cloudflare/vitest-pool-workers/config";
+
+export default defineWorkersConfig({
+	test: {
+		deps: {
+			optimizer: {
+				ssr: {
+					enabled: true,
+					include: ["your-package-name"],
+				},
+			},
+		},
+		poolOptions: {
+			workers: {
+				// ...
+			},
+		},
+	},
+});
+```
+
+Fixed #6591, #6581, #6405.

fixtures/vitest-pool-workers-examples/external-package-resolution/test/index.spec.ts

@@ -0,0 +1,34 @@
+# ⚡ module-resolution
+
+This fixture demonstrates that the Vitest integration correctly resolves modules, including:
+
+- A CommonJS package that requires a directory rather than a specific file.
+- A package without a main entrypoint or with browser field mapping, handled via [Dependency Pre-Bundling](#dependency-pre-bundling).
+
+## Dependency Pre-Bundling
+
+[Dependency Pre-Bundling](https://vite.dev/guide/dep-pre-bundling) is a Vite feature that converts dependencies shipped as CommonJS or UMD into ESM. If you encounter module resolution issues—such as: `Error: Cannot use require() to import an ES Module` or `Error: No such module`—you can pre-bundle these dependencies using the [deps.optimizer](https://vitest.dev/config/#deps-optimizer) option:
+
+```ts
+import { defineWorkersConfig } from "@cloudflare/vitest-pool-workers/config";
+
+export default defineWorkersConfig({
+	test: {
+		deps: {
+			optimizer: {
+				ssr: {
+					enabled: true,
+					include: ["your-package-name"],
+				},
+			},
+		},
+		poolOptions: {
+			workers: {
+				// ...
+			},
+		},
+	},
+});
+```
+
+See our [vitest config](./vitest.config.ts) for an example of how we pre-bundled `discord-api-types/v10` and `@microlabs/otel-cf-workers`.

fixtures/vitest-pool-workers-examples/external-package-resolution/test/tsconfig.json

@@ -1,4 +1,10 @@
 import { Toucan } from "toucan-js";
+// Testing dependency without a main entrypoint
+// @see https://github.com/cloudflare/workers-sdk/issues/6591
+import "discord-api-types/v10";
+// Testing dependency with browser field mapping
+// @see https://github.com/cloudflare/workers-sdk/issues/6581
+import "@microlabs/otel-cf-workers";
 
 export default {
 	async fetch(): Promise<Response> {