ESQL: Clearer error message on nested aggregations

[GalLalouche]

This PR replaces an internal error ("plan optimized incorrectly due to missing references") that happens with nested aggregations (e.g., count inside of average) with a proper 400 error, so the user would know what they did wrong, and this won't get flagged up as an internal error for on-callers.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[elasticsearchmachine]

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

[github-actions]

🔍 Preview links for changed docs

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[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

[MattAlp]

The PR body itself could use a bit more detail - LGTM content-wise