Add Existing External AI Search Index

Allow adding Existing External AI Search Index to group (and eventually public) workspaces.

Will support citations using the chunk but not enhanced (showing the file versus showing the chunk content).
Will NOT support individual document selection because there is not parent document details anywhere.

Best path forward is to create a semantic kernel plugin for this

The issue names no files or tests; start by locating the existing Semantic Kernel plugin integration and the workspace and external-index entry points. Use the stated group and public workspace scope, citation limitations, and absence of individual document selection to define the acceptance criteria.

Written by the indexing model from the issue text.