"could not read file as String: devtools_extensions/..." if pointing to different version of package without extension present

This is splitting off a separate issue from #8912 from https://github.com/flutter/devtools/issues/8912#issuecomment-2672370477 based on discussion in comment https://github.com/flutter/devtools/issues/8912#issuecomment-2679871965.

If you point to a version of a package that has a devtool extension, then point to another version that does not have that extension present (i.e. built), you get an error about "could not read file".

> If you point to a version of rohd which has a devtools extension present (e.g. the artifacts branch), and get it loaded up successfully, it then adds to the devtools_options.yaml file:
>
> description: This file stores settings for Dart & Flutter DevTools.
> documentation: https://docs.flutter.dev/tools/devtools/extensions#configure-extension-enablement-states
> extensions:
> - rohd: true
> If you then point to a version of rohd which does not have the devtools extension (e.g. the main branch), then when you open devtools, the rohd tab is still present but you get an error
>
> could not read file as String: devtools_extensions/rohd_0.0.1/index.html
>
> 
>
> Is that the intended error message when an extension is not present?

通过加载带有 DevTools 扩展的包版本，然后切换到不带内置扩展的版本来复现该问题。首先跟踪扩展文件缺失时，已保存的 devtools_options.yaml 扩展条目是如何处理的。当缺失扩展时的行为和错误消息是有意设计的，并且已由回归测试覆盖或得到明确改进时，即表示完成。

由索引模型根据 Issue 内容生成。