Sets 'separators' and 'separators_group' parameters to optional and updates descriptions

[kosabogi]

This PR updates the InferenceChunkingSettings type by making the separator_group and separators fields optional.
These fields are only applicable to the recursive strategy and required when using it.
Updated the field descriptions to clarify their usage.

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[github-actions]

The backport to 9.0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-9.0 9.0
# Navigate to the new working tree
cd .worktrees/backport-9.0
# Create a new branch
git switch --create backport-5216-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ac89683f5d6a90d5a40e12537fd61a8d1130fbe2
# Push it to GitHub
git push --set-upstream origin backport-5216-to-9.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.0

Then, create a pull request where the base branch is 9.0 and the compare/head branch is backport-5216-to-9.0.