fix: allow to enable lazy compilation via Rspack config

[chenjiahan]

Summary

Rsbuild now checks the compiler options to determine whether lazy compilation is enabled.

Rsbuild users can enable lazy compilation in two ways:

1. Use Rsbuild's dev.lazyCompilation option
2. Use Rspack's experiments.lazyCompilation option (see the added test case)

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR updates the dev middleware to detect lazy compilation from Rspack’s experiments.lazyCompilation setting (in addition to Rsbuild’s own dev.lazyCompilation), and adds E2E tests to verify both configuration paths.

• core: replace direct dev.lazyCompilation check with a helper that reads experiments.lazyCompilation (and should also fall back to dev.lazyCompilation)
• e2e: remove hard-coded dev.lazyCompilation in config and add tests for both Rsbuild and Rspack experiment flags

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/core/src/server/devMiddlewares.ts	Add isLazyCompilationEnabled helper using Rspack experiments and wire up middleware
e2e/cases/lazy-compilation/basic/rsbuild.config.ts	Remove legacy dev.lazyCompilation from sample config
e2e/cases/lazy-compilation/basic/index.test.ts	Add tests for both Rsbuild dev.lazyCompilation and tools.rspack.experiments.lazyCompilation

Comments suppressed due to low confidence (3)

packages/core/src/server/devMiddlewares.ts:69

• You removed the dev parameter from the middleware setup, but it’s needed to read dev.lazyCompilation. Consider re-adding dev to the destructure so it can be passed into your lazy check.

const applyDefaultMiddlewares = async ({

e2e/cases/lazy-compilation/basic/rsbuild.config.ts:9

• [nitpick] The sample config no longer includes dev.lazyCompilation; consider adding a second E2E variant that exercises the legacy flag in the actual config file, not just via the dev() call override.

      page2: './src/page2/index.js',

packages/core/src/server/devMiddlewares.ts:135

• The helper only checks experiments.lazyCompilation, but Rsbuild’s own dev.lazyCompilation was removed from the condition. Re-include dev.lazyCompilation (from the original dev parameter) in this function so both flags enable lazy compilation.

    const isLazyCompilationEnabled = () => {