Fix the __name__ normalization for vector index

[ninjaAB-5]

Description

Vector index of standard edition database are not properly handled when it comes to __name__ normalization, which can cause issues like #9385. This PR is to address that issue.

Scenarios Tested

A reproduce of #9385 -

1. Fill in firestore.indexes.json with

{
  "indexes": [
    {
      "collectionGroup": "myVectorCollection",
      "queryScope": "COLLECTION",
      "fields": [
        {
          "fieldPath": "embedding",
          "vectorConfig": {
            "dimension": 768,
            "flat": {}
          }
        }
      ],
      "density": "SPARSE_ALL"
    }
  ],
  "fieldOverrides": []
}

2. Run firebase deploy --only firestore:indexes and successfully deploy the index
3. Pull the latest index definition via firebase firestore:indexes --database=index-fix > firestore.indexes.json and confirm the normalization is properly handled.

{
  "indexes": [
    {
      "collectionGroup": "myVectorCollection",
      "queryScope": "COLLECTION",
      "fields": [
        {
          "fieldPath": "__name__",
          "order": "ASCENDING"
        },
        {
          "fieldPath": "embedding",
          "vectorConfig": {
            "dimension": 768,
            "flat": {}
          }
        }
      ],
      "density": "SPARSE_ALL"
    }
  ],
  "fieldOverrides": []
}

4. Deploy again with firebase deploy --only firestore:indexes and no error shows up.

See the test added to src/firestore/indexes.spec.ts for other scenarios tested.