Copilot chat shows ignored-files troubleshooting text in grep search tool completion UI

- Copilot Chat Extension Version: 0.57.0
- VS Code Version: 1.128.0
- Commit: fc3def6774c76082adf699d366f31a557ce5573f
- OS Version: Darwin arm64 25.5.0
- Feature (e.g. agent/edit/ask mode): any with grep search tool allowed
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): any model that uses the search tool
- Logs: (none attached)

**Steps to Reproduce:**

1. Open a workspace where the target files are excluded by search.exclude or ignore files.
2. In Copilot Chat agent mode, ask Copilot to search for text that will return no matches.

**Expected:**

The tool completion UI shows a brief user-facing no-results message.

**Actual:**

The chat UI shows internal troubleshooting guidance:

> No matches found. Your search pattern might be excluded completely by either the search.exclude settings or .*ignore files. [...]

**Additional Context:**

This is user-visible in chat even though the text reads like agent guidance rather than a user-facing result message. This seems to have changed recently - previous "No matches found" did not show this guidance, likely due to the recent defaults change in https://github.com/microsoft/vscode/pull/324186.