Refactor: use Schema.walk() in AdaptFrameworkImport.extractAssets instead of manual recursion

[taylortom]

AdaptFrameworkImport.extractAssets() manually recurses through schema properties and data to find Asset-type fields and remap their values via this.assetMap. This duplicates the traversal logic already provided by Schema.walk().

The call site already has a schema instance:

this.extractAssets(schema.built.properties, insertData)

Could be refactored to:

extractAssets (schema, data) {
  if (!schema) return
  const matches = schema.walk(data, field =>
    field?._backboneForms?.type === 'Asset' || field?._backboneForms === 'Asset'
  )
  matches.forEach(({ data: parent, key, value }) => {
    value !== '' ? parent[key] = this.assetMap[value] ?? value : delete parent[key]
  })
}

This eliminates the manual recursion and uses the shared schema walking infrastructure.

See also: adapt-security/adapt-authoring-courseassets#40 (same pattern for extractAssetIds)