Fix invalid locale issue

[HiDeoo]

Description

This PR fixes a build issue visible in withastro/docs#9890

For project without a root locale like the docs, for pages not matching the default locale slug, we were not falling back to the default locale.

I also tested the fix locally on the failing Astro Docs branch and it now builds properly.

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Latest commit: 6c426de

The changes in this PR will be included in the next version bump.

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@astrojs/starlight	Patch

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[HiDeoo]

@delucis Just realized the fix is wrong, please hold on merging

[delucis]

@HiDeoo said the fix is wrong!

[HiDeoo]

To explain with a bit more details, I just realized we actually had 2 implementations of slugToLocale(), 1 tested and the other one not tested, and this PR only updated 1 of them.

I just pushed new changes removing the duplicated code and both code paths now use the same underlying implementation which ends up being tested.

I think there could be some potential reorganization of the code needed, e.g. packages/starlight/integrations/shared/slugToLocale.ts is not only used by integrations. We could do that later in a separate PR or now if preferred but I don't think I now have a clear idea on the structure/naming we could use.

[delucis]

Thanks for catching the duplication!

[delucis]

I think there could be some potential reorganization of the code needed, e.g. packages/starlight/integrations/shared/slugToLocale.ts is not only used by integrations.

I guess it will depend what else is using these? But for sure I think we can release this fix and refactor later.