fix: fall back to dev schema for dev MEI versions

[yinanazhou]

Verovio was updated and now defaults to outputting 6.0-dev. When Neon tries to fetch the corresponding schema for validation, it constructs the URL using the mei version attribute. However, the music-encoding schema repository does not have a 6.0-dev folder. The dev schema lives under dev/. This causes the fetch to 404, and Neon surfaces "Unknown MEI Version" to the user.

Since there is no way to set the version Verovio outputs, the fix lives on the schema-fetching side in Neon. fetchSchema now tries the exact mei version value first. If that fetch fails and the version contains dev, it retries with dev as the schema folder. This preserves forward compatibility: if a future dev version gets its own dedicated folder on the schema repo, it will be used automatically; only versions without a matching folder fall back to dev.

refs: #1356

[yinanazhou]

Merging as the broken link is due to cirmmt website is getting 502 error.