Linear retriever top level option for normalizer

[mridula-s109]

Summary

Adds a top-level normalizer option to the linear retriever.
The top-level normalizer acts as the default for all sub-retrievers, but each sub-retriever can override it if needed.

Behavior

Normalizer resolution priority:

1. Retriever-specific normalizer (if specified)
2. Top-level normalizer (if specified)
3. None (default)

Example

GET /products/_search
{
  "retriever": {
    "linear": {
      "normalizer": "minmax",
      "retrievers": [
        {
          "retriever": {
            "standard": {
              "query": {
                "term": {"category": "electronics"}
              }
            }
          },
          "weight": 2.0
        },
        {
          "retriever": {
            "standard": {
              "query": {
                "range": {"rating": {"gte": 4.5}}
              }
            }
          },
          "weight": 1.0,
          "normalizer": "l2_norm"
        }
      ]
    }
  },
  "_source": ["name", "category", "rating", "price"]
}

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[elasticsearchmachine]

Hi @mridula-s109, I've created a changelog YAML for you.

[github-actions]

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[ioanatia]

nice work and good test coverage - left some comments, most of them minor

[Mikep86]

Partial review, covered everything except the documentation. Looks good overall! I left a few minor comments.

[mridula-s109]

@ioanatia @Mikep86 Appreciate the feedback! I’ve made the changes and marked them as resolved. Let me know if you spot anything else. @kderusso tagging you as well since your requested changes still need approval.

[kderusso]

Nice work @mridula-s109 - I left a few minor comments, but this is very close!

[mridula-s109]

Nice work @mridula-s109 - I left a few minor comments, but this is very close!

Thanks for your comments @kderusso , have resolved all the cluster features comments and updated the documentation.

[kderusso]

Thanks for making the changes @mridula-s109 - I've left some nitpicky comments, and you may need to update/there are some failing CI tests that look unrelated. Otherwise LGTM. Nice work, thanks for iterating!

[mridula-s109]

@Mikep86 Can you please have a look at it as you had requested changes as well?

[Mikep86]

Great job cleaning this up! I left a couple non-nit comments about the some missing applies_to tags in the docs. The rest is non-blocking.

Breaking changes removed

[mridula-s109]

Great job cleaning this up! I left a couple non-nit comments about the some missing applies_to tags in the docs. The rest is non-blocking.

Thanks for the comments have resolved them all.