fix(preset): keep lazy handlers code-split in dev

[jibin7jose]

Description

This PR fixes an issue where an import-time error in a single lazy route handler takes down the entire dev server and breaks unrelated routes with a misleading Cannot access '<x>' before initialization error.

Cause:
The nitro-dev preset was setting inlineDynamicImports: true. This caused Rollup to inline all lazy handler bodies into the single index.mjs startup bundle. An error evaluating one route would abort the execution of the bundle, leaving subsequent routes permanently in the Temporal Dead Zone (TDZ).

Fix:
We've set inlineDynamicImports: false in the nitro-dev preset. This ensures lazy handlers remain code-split during local development, so an import-time failure in one module will not disrupt the evaluation of the rest of the server or other routes. When a broken route is accessed, the dev server will now accurately report the original initialization error.

Fixes #4435

[vercel]

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 35ff2855-fe37-4fc4-8078-31048668a773

📥 Commits

Reviewing files that changed from the base of the PR and between a217da5 and a6ab1a4.

📒 Files selected for processing (4)

• src/presets/_nitro/nitro-dev.ts
• test/fixtures/dev-import-error/routes/boom.get.ts
• test/fixtures/dev-import-error/routes/ok.get.ts
• test/unit/dev-import-error.test.ts

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📝 Walkthrough

Walkthrough

The Nitro dev preset now keeps dynamic imports un-inlined and updates related preset configuration. Regression fixtures and a Vitest suite verify that an import-time error in one route does not prevent an unrelated route from responding.

Changes

Nitro dev import error handling

Layer / File(s)	Summary
Update Nitro dev preset src/presets/_nitro/nitro-dev.ts	Updates preset imports and paths, disables dynamic import inlining, changes external tracing and source map options, and removes the custom task handler.
Add import-error regression coverage test/fixtures/dev-import-error/routes/*, test/unit/dev-import-error.test.ts	Adds healthy and import-failing routes and verifies their independent dev-server responses, error content, reload setup, and cleanup.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses a valid conventional commits format and summarizes the dev code-splitting fix.
Description check	✅ Passed	The description clearly matches the dev-server lazy-handler import error fix.
Linked Issues check	✅ Passed	The changes implement #4435 by keeping lazy handlers code-split in dev and adding a regression test.
Out of Scope Changes check	✅ Passed	The additional preset and test fixture changes stay within the lazy-handler dev-server fix scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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