fix(config): default output.module to true when using module library type - #22063

Summary

Following the removal of experiments.outputModule in #22011, output.module should default to true whenever an ESM output is configured.

Previously, output.module only defaulted to true when targeting universal, deno, or bun (tp.node === null && tp.web === null && tp.module !== false, tp.deno === true, tp.bun === true). If a user configured output.library.type: "module" or "modern-module" (or set it per-entry in entry[name].library.type: "module"), output.module remained false.

As a consequence:

1. output.iife defaulted to true (because !output.module).
2. Webpack wrapped the output in an IIFE (() => { ... })().
3. ModuleLibraryPlugin emitted export default ... inside the IIFE body:

   (() => {
   // ...
   export default __webpack_exports__;
   })();

4. This emitted invalid JavaScript syntax, throwing a runtime SyntaxError: Unexpected token 'export' and failing Terser minification in production ("Export" statement may only appear at the top level).
5. Furthermore, configuring output: { module: false, library: { type: "module" } } was not guarded, allowing conflicting options to produce invalid code silently.

This PR:

• Defaults output.module to true in lib/config/defaults.js when output.library.type or any entry[name].library.type is "module" or "modern-module".
• Adds an explicit error in lib/WebpackOptionsApply.js when !output.module but library type "module" or "modern-module" is enabled: library type "<type>" is only allowed when 'output.module' is enabled.
• Updates test/harness/runner/index.js so .mjs runner tests with module library types are correctly dispatched to ESM execution.

Fixes #22061.

What kind of change does this PR introduce?

bugfix

Did you add tests for your changes?

Yes:

• Unit tests in test/Defaults.unittest.js testing that output.module defaults to true for output.library.type: "module", "modern-module", and entry[name].library.type: "module", and tests that a descriptive error is thrown when output.module: false with module library types.
• Integration config test in test/configCases/library/module-default-output-module/ verifying compilation and ESM execution without manually setting output.module: true.

Does this PR introduce a breaking change?

No.

If relevant, what needs to be documented once your changes are merged or what have you already documented?

Documentation for output.library.type: "module" can note that output.module is automatically enabled.

Use of AI

Yes

Summary by CodeRabbit

• New Features
  • Module output is now enabled by default when using the module or modern-module library types, including per-entry libraries.
  • ES module libraries can now be used without explicitly setting module output.
• Bug Fixes
  • Added clear validation errors when module library types are configured with module output disabled.
  • Improved handling of .mjs library bundles so they run correctly as ES modules in supported environments.