Enable early termination for HNSW by default

[tteofili]

This enables early termination for KNN queries running on HNSW by default.
See #127223

[elasticsearchmachine]

Hi @tteofili, I've created a changelog YAML for you.

[tteofili]

openai_vector comparison:
4886... is with early_termination enabled
4718... is with early termination disabled (current default)

[github-actions]

🔍 Preview links for changed docs:

• docs/reference/elasticsearch/index-settings/index-modules.md

🔔 The preview site may take up to 3 minutes to finish building. These links will become live once it completes.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/elasticsearch/index-settings/index-modules.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.

We use applies_to tags to mark version-specific features and changes.

Expand for a quick overview

When to use applies_to tags:

✅ At the page level to indicate which products/deployments the content applies to (mandatory)
✅ When features change state (e.g. preview, ga) in a specific version
✅ When availability differs across deployments and environments

What NOT to do:

❌ Don't remove or replace information that applies to an older version
❌ Don't add new information that applies to a specific version without an applies_to tag
❌ Don't forget that applies_to tags can be used at the page, section, and inline level

🤔 Need help?

• Check out the cumulative docs guidelines
• Reach out in the #docs Slack channel

[tteofili]

Buildkite benchmark this with so-vector please

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 5a04cf2
• Baseline: 1b66a2b (env ID 75875010-35c6-44fd-8728-27497efff698)
• Contender: 5a04cf2 (env ID eac0e095-a3c8-4d80-9028-597137f800ea)
• Benchmark results

This build ran two so-vector benchmarks to evaluate performance impact of this PR.

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