Agent does not receive search/codebase in its tool definitions.

- Copilot Chat Extension Version: 0.63.0
- VS Code Version: 1.135.0
- OS Version: Windows_NT x64 10.0.26200 (build 26200.9168)
- Feature (e.g. agent/edit/ask mode): Local Agent mode, custom agents, subagents, and semantic codebase search
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet):
- GPT-5.6 Sol (ChatGPT) (`gcmp.codex`)
- DeepSeek-V4-Flash (`gcmp.deepseek`)
- Logs:
- `#codebase` explicitly invoked `semantic_search` successfully:
```text
timestamp: 2026-08-31 12:00:55 +08:00
type: tool_call
name: semantic_search
status: ok
duration: 3893 ms
query: "#codebase test"
```
- The model tool snapshots did not contain any of the following names:
```text
semantic_search
copilot_searchCodebase
search/codebase
```
- When the Explore custom agent was explicitly instructed to use only semantic codebase search, no semantic search tool call was emitted. The subagent responded:
```text
The semantic codebase search tool was not provided in the current tool list.
```
- The same result occurred when requesting the tool using each of these forms:
```text
search/codebase
semantic_search
"the natural-language semantic workspace search tool"
```

## Description

The semantic codebase search tool is enabled in the tools picker and/or configured in a custom agent, but it is not included in the actual tool definitions provided to the model.

This affects both:

1. The main Agent session when the Codebase tool is selected.
2. A custom agent invoked through `runSubagent`, even when its frontmatter explicitly includes the `search` tool set or `search/codebase`.

However, explicitly entering `#codebase` in the user prompt works correctly and invokes `semantic_search`. This confirms that semantic workspace search is available and functional for the workspace, but it is not exposed as an autonomous agent tool.