"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?

Reproduce the issue by loading a package version with a DevTools extension and then switching to a version without the built extension. Start by tracing how the saved devtools_options.yaml extension entry is handled when the extension files are missing. Done means the missing-extension behavior and error message are intentional and covered by a regression test or clearly improved.

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