…ngGraph code paths for `8.15` (elastic#188168)

## Summary

This PR enables the automatic setup of the Knowledge Base and LangGraph
code paths for the `8.15` release. These features were behind the
`assistantKnowledgeBaseByDefault` feature flag, which will remain as a
gate for upcoming Knowledge Base features that were not ready for this
release.

As part of these changes, we now only support the new LangGraph code
path, and so were able to clean up the non-kb and non-RAGonAlerts code
paths. All paths within the `post_actions_executor` route funnel to the
LangGraph implementation.

> [!NOTE]
> We were planning to do the switch to the new
[`chat/completions`](https://github.com/elastic/kibana/pull/184485/files)
public API, however this would've required additional refactoring since
the API's slightly differ. We will make this change and delete the
`post_actions_executor` route for the next release.

### Checklist

Delete any items that are not applicable to this PR.

- [X] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- Working with docs team to ensure updates before merging, cc
@benironside
- [X] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

---------

Co-authored-by: Steph Milovic <stephanie.milovic@elastic.co>
(cherry picked from commit 661c251)

# Conflicts:
#	.buildkite/ftr_configs.yml