Integrate weights into simplified RRF retriever syntax

[mridula-s109]

PR Description:

Summary

Integrates per-field weight support into the simplified RRF retriever syntax, allowing users to specify field boosting using the field^weight notation.

Changes Made

• Enhanced RRF Retriever Builder: Modified RRFRetrieverBuilder to parse and apply per-field weights from the simplified field syntax
• Weight Validation: Added validation to ensure weights are non-negative (zero is allowed)
• Feature Flag: Introduced SIMPLIFIED_WEIGHTED_SUPPORT node feature for backward compatibility
• Comprehensive Testing: Added parsing tests, validation tests, and functional tests for various weight scenarios

Usage Example

Users can now specify weighted fields in the simplified RRF syntax:

GET /restaurants/_search
{
  "retriever": {
    "rrf": {
      "query": "pizza",
      "fields": ["name^2.0", "description^0.5", "tags"]
    }
  }
}

[github-actions]

🔍 Preview links for changed docs

• docs/reference/query-languages/esql/kibana/docs/functions/absent.md
• docs/reference/query-languages/esql/kibana/docs/functions/absent_over_time.md
• docs/internal/Versioning.md
• docs/reference/elasticsearch/command-line-tools/certgen.md
• docs/reference/elasticsearch/command-line-tools/certutil.md
• docs/reference/elasticsearch/command-line-tools/create-enrollment-token.md
• docs/reference/elasticsearch/command-line-tools/elasticsearch-croneval.md
• docs/reference/elasticsearch/command-line-tools/elasticsearch-keystore.md
• docs/reference/elasticsearch/command-line-tools/index.md
• docs/reference/elasticsearch/command-line-tools/node-tool.md

More links …

• docs/reference/elasticsearch/command-line-tools/reconfigure-node.md
• docs/reference/elasticsearch/command-line-tools/reset-password.md
• docs/reference/elasticsearch/command-line-tools/saml-metadata.md
• docs/reference/elasticsearch/command-line-tools/service-tokens-command.md
• docs/reference/elasticsearch/command-line-tools/setup-passwords.md
• docs/reference/elasticsearch/command-line-tools/shard-tool.md
• docs/reference/elasticsearch/command-line-tools/syskeygen.md
• docs/reference/elasticsearch/command-line-tools/users-command.md
• docs/reference/elasticsearch/configuration-reference/discovery-cluster-formation-settings.md
• docs/reference/elasticsearch/rest-apis/retrievers/linear-retriever.md
• docs/reference/query-languages/esql/esql-lookup-join.md
• docs/reference/query-languages/esql/functions-operators/aggregation-functions.md
• docs/reference/query-languages/esql/kibana/docs/functions/decay.md
• docs/reference/query-languages/esql/kibana/docs/functions/replace.md
• docs/reference/query-languages/esql/kibana/docs/functions/std_dev.md
• docs/reference/query-languages/query-dsl/query-dsl-minimum-should-match.md
• docs/release-notes/breaking-changes.md
• docs/release-notes/deprecations.md
• docs/release-notes/index.md

[github-actions]

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

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• Check out the cumulative docs guidelines
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[mridula-s109]

Thanks for the feedback @ioanatia @Mikep86 @dimitris-athanasiou 🙌 I’ve made the changes and addressed all comments. Let me know if there’s anything further you’d like me to tweak.

[dimitris-athanasiou]

LGTM