feat(server): add doc keyword search tool (#12837)

close AI-185

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Introduced a keyword-based document search tool, allowing users to
search for relevant documents within their workspace using keywords.
- Search results include document titles, summaries, and direct links,
enhancing document discovery and navigation.
- **Bug Fixes**
  - None.
- **Tests**
- Added new tests to verify document search by IDs and by keywords,
ensuring accurate and reliable search functionality.
- **Documentation**
  - None.
- **Chores**
- Updated configuration file organization for improved clarity; no
changes to functionality.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->








#### PR Dependency Tree


* **PR #12867**
  * **PR #12863**
    * **PR #12837** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

packages/backend/server/src/plugins/copilot/index.ts

@@ -8,6 +8,7 @@ import { FeatureModule } from '../../core/features';
 import { PermissionModule } from '../../core/permission';
 import { QuotaModule } from '../../core/quota';
 import { WorkspaceModule } from '../../core/workspaces';
+import { IndexerModule } from '../indexer';
 import {
   CopilotContextResolver,
   CopilotContextRootResolver,
@@ -44,6 +45,7 @@ import {
     PermissionModule,
     ServerConfigModule,
     WorkspaceModule,
+    IndexerModule,
   ],
   providers: [
     // providers