[BUG] usage of nested knn query and efficient filter with nested query is not correct

[wdongyu]

What is the bug?
From #1356, when we need to execute a nested knn query and an efficient filter with a nested field, we are advised to use query like this:

{
  "query": {
    "nested": {
      "path": "test_nested",
      "query": {
        "knn": {
          "test_nested.test_vector": {
            "vector": [
              5
            ],
            "k": 24,
            "filter": {
              "nested": {
                "path": "test_nested",
                "query": {
                  "term": {
                    "test_nested.parking": "false"
                  }
                }
              }
            }
          }
        }
      },
      "inner_hits": {}
    }
  }
}'

But indeed, it will return wrong number of hits. When I use the following query, it works as expected:

{
  "query": {
    "nested": {
      "path": "test_nested",
      "query": {
        "knn": {
          "test_nested.test_vector": {
            "vector": [
              5
            ],
            "k": 24,
            "filter": {
              "term": {
                "test_nested.parking": "false"
              }
            }
          }
        }
      },
      "inner_hits": {}
    }
  }
}

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Base on testcase AdvancedFilteringUseCasesIT.testFiltering_whenNestedKNNAndFilterFieldWithNestedQueries_thenSuccess, we first increase k to 100 as we need to match more nested knn docs in all(NUM_DOCS=50) parent docs:

private static final int k = 100;

2. In final validation, change totalSearchHits to NUM_DOCS/2 = 25, as there are half docs that meet the requirements:

Assert.assertEquals("For engine " + engine + ", totalSearchHits: ", NUM_DOCS / 2, parseTotalSearchHits(response));
if (KNNEngine.getEngine(engine) == KNNEngine.FAISS) {
    ...
    Assert.assertEquals("For engine " + engine + ", totalSearchHits with ANN search :", NUM_DOCS / 2, parseTotalSearchHits(response));
    ...
}

3. Run the test and it fail, but other tests in the same IT pass:

 totalSearchHits:  expected:<25> but was:<24>

What is the expected behavior?
Nested query in efficient filter should work.

What is your host/environment?

• OS: Debian GNU/Linux 8
• Version: 2.18.0
• Plugins: KNN

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