[BUG] OS 1.3.7 -> OS 2.19 transformation error TextNode cannot be cast to ObjectNode

[ragu2003]

What is the bug?

When we do console metadata evaluate we get the following error
ERROR - test_index failed to transform to the target version: Transformation for index index test_index failed due to class com.fasterxml.jackson.databind.node.TextNode cannot be cast to class com.fasterxml.jackson.databind.node.ObjectNode
(com.fasterxml.jackson.databind.node.TextNode and com.fasterxml.jackson.databind.node.ObjectNode are in unnamed module of loader 'app')

What are your migration environments?

Source: OS 1.3.7
Target: OS 2.19
MigrationAssistant: 2.5.0

How can one reproduce the bug?

Create an index with the following setting:

{
    "settings": {
      "index": {
        "number_of_shards": "1",
        "provided_name": "test-migrations-reindex-fixed",
        "knn.space_type": "cosinesimil",
        "knn": "true",
        "creation_date": "1757078780188",
        "number_of_replicas": "1",
        "uuid": "_5TwzpZUQKa37bAjmf6EAQ",
        "version": {
          "created": "135248527",
          "upgraded": "136407827"
        }
      }
    }
}

What is the expected behavior?

Migration assistant should transform this correctly and send this for indexing.

Do you have any additional context?

I suspect it is happening because of these two settings:

"knn.space_type": "cosinesimil",
"knn": "true"

Migration assistant tries to construct a map like this:
knn: {
space_type: cosinesimil
}
When it reads "knn": "true", the textnode collides with the object node.

[peternied]

@gregschohn Could you look into this and figure out how to support this scenario around knn index settings?