Make isValidNewName route always return 200

[frcroth]

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• click on "Add Dataset" in dashboard
• type a name into the "Dataset Name" field that already exists for another ds -> red label should appear
• change name to something unique -> error should disappear
• no error toast should appear during the above

TODOs:

• Since it is not user facing, no Changelog, only migration?

Issues:

• fixes dataset isValidNewName route should always return 200, with true/false body #7399

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(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Updated migration guide if applicable

[fm3]

Backend LGTM :) @philippotto could you have a look at the frontend (and fix it if needed)? My guess is that showErrorToast: false is no longer needed, since in normal operation there should no longer be an error 400 here.

@frcroth I think it would be nice to add a short changelog entry, if only for ourselves to note that the API version changed here. Also I’m afraid I caused some small merge conflicts with #7552 😅

[philippotto]

sure! I pushed a small commit. testing went well 🚀