fix(file_read): return error message when search finds no results

[wangwangbobo]

Summary

When search_file returns empty results, the tool was returning an empty content list {"content": []} with status: "success". This causes a Bedrock API error because tool_use IDs must have corresponding tool_result blocks.

Fix

Added a check after search_file() — if results are empty, append a clear error message instead of extending with empty results.

if not results:
    response_content.append({"text": "No search results found for the given pattern."})
else:
    response_content.extend([{"text": r["context"]} for r in results])

Related Issue

Fixes #303