fix: handle empty results in MCP keyword_search tool (#15058)

## Description

Fixes: #15038 — MCP keyword_search tool errors with "Unexpected response
type" when no results are found.

### Problem

When the MCP `keyword_search` tool returns no matching documents, the
access control `.docs()` method may return `undefined`/`null` for an
empty input array. Calling `.map()` on this value throws an error, and
the MCP framework wraps it as "Unexpected response type".

### Solution

Added a guard check after the permission filtering step. If the result
is empty or null, the tool now returns a proper informational response
instead of throwing.

### Changes

- `packages/backend/server/src/plugins/copilot/mcp/provider.ts`: Added
null/empty check before `docs.map()` in the keyword_search tool execute
function.

### Testing

- **Before**: `keyword_search` with a non-existent keyword throws
"Unexpected response type"
- **After**: `keyword_search` with a non-existent keyword returns `{
content: [{ type: 'text', text: 'No matching documents found.' }] }`


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

* **Bug Fixes**
* Prevented errors when document data is missing, improving search
stability.
* Improved search feedback by displaying a clear "No matching documents
found." message instead of empty results.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->