Move indexed content storage from config to database

Problem/Motivation

Currently, indexed content is stored in the ai_content_chat.settings config file under the indexed_content key. This approach has several problems:
Config file bloat – As more content is indexed, the config file grows significantly, potentially reaching megabytes for larger sites
Config export issues – Large config files slow down drush cex/cim and version control operations
Not scalable – Sites with thousands of nodes will experience performance degradation
Config vs content – Indexed content is dynamic data, not configuration; storing it in config violates Drupal's config/content separation principle

Proposed resolution

Create a dedicated database table to store indexed content

Data model changes

New database table: ai_content_chat_index
Remove indexed_content from ai_content_chat.settings config schema