fix(pack): invalid css url request should throw waring instead of error

[fireairforce]

Summary

upstream pr: utooland/next.js#105

closes: #2278

Test Plan

添加了快照测试。

[gemini-code-assist]

Summary of Changes

Hello @fireairforce, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the build process's handling of invalid CSS url() requests. Previously, such issues would halt the build with an error; now, they will gracefully proceed while issuing a warning. This change aims to enhance developer experience by preventing unnecessary build failures while still providing crucial feedback on potential styling issues. The update is validated through new snapshot tests and includes a relevant dependency update.

Highlights

• Error Handling Improvement: Invalid CSS url() requests, such as those with empty strings or unresolvable paths, will now emit a warning instead of causing a build error. This change improves the robustness of the build process.
• Snapshot Tests Added: New snapshot tests have been introduced to verify that the build system correctly generates warnings for invalid url() values in CSS background-image properties, ensuring the intended behavior is maintained.
• Dependency Update: The next.js subproject commit has been updated, likely incorporating the core changes that enable the new warning-based behavior for invalid CSS url() resolutions.

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[gemini-code-assist]

Code Review

This pull request correctly implements the intended change of converting an error into a warning for invalid CSS url() requests. This is supported by the addition of new snapshot tests that demonstrate the new warning behavior. My review includes a suggestion to correct the CSS syntax in one of the new test cases to ensure the test is precise and avoids unrelated CSS invalidity.