Metadata validation behavior diverges for non-current schema versions

See https://github.com/dandi/dandi-archive/issues/2687#issuecomment-3893988134

When validating the metadata of a dandiset with a schema version that is older than the current schema version, instead of being constructed locally from the models (and crucially, supplied env vars) within dandischema, it is [fetched](https://github.com/dandi/dandi-schema/blob/07aa22c391c0135bc9275d515de71eae831b5c41/dandischema/metadata.py#L198) from the generated dandi [schema](https://github.com/dandi/schema/tree/master) files.

Read dandischema/metadata.py around line 198 and the linked issue discussion, then compare that path with the generated dandi/schema files. Done means validation for older schema versions uses locally constructed dandischema models with supplied environment variables instead of fetching generated schema files.

Written by the indexing model from the issue text.