perf(dev-server): make asset middleware file lookup async

[chenjiahan]

Summary

• use async stats from Rspack output fs to avoid sync bottlenecks
• simplify OutputFileSystem typing

Checklist

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

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[Copilot]

Pull Request Overview

This PR optimizes the asset middleware by converting synchronous file system operations to asynchronous ones to avoid blocking the event loop. The main goal is to improve performance by using async stats from Rspack's output filesystem instead of sync bottlenecks.

Key changes:

• Convert getFileFromUrl function from synchronous to asynchronous
• Update OutputFileSystem type definition to use Rspack's native types
• Update all callers to properly await the async file lookups

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/core/src/server/assets-middleware/getFileFromUrl.ts	Convert to async function with Promise-based file stat operations
packages/core/src/server/assets-middleware/middleware.ts	Update caller to await the async getFileFromUrl function
packages/core/src/server/browserLogs.ts	Update caller to await the async getFileFromUrl function
packages/core/src/server/assets-middleware/index.ts	Update OutputFileSystem type to use Rspack's native types

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