[error] Failed to compute embeddings for tools: Error: Error fetching embeddings: 422

Copilot tool search is failing with:
```
[error] Failed to compute embeddings for tools: Error: Error fetching embeddings: 422
```

This is resulting in explicitly named tools provided by other extensions that are enabled in the copilot session to be uncallable by the agent because tool search cannot find it by name.

- Copilot Chat Extension Version:
- VS Code Version: 1.137.0-insider
- OS Version: Ubuntu 26.04.1 LTS
- Feature (e.g. agent/edit/ask mode): Agent
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): GPT-5.6 Luna
- Logs:

- Output panel > GitHub Copilot Chat: [GitHub Copilot Chat.log](https://github.com/user-attachments/files/31750001/GitHub.Copilot.Chat.log)

- Run Command: "Developer: Show Chat Debug View": [_toolSearch__problems__logs.json](https://github.com/user-attachments/files/31750027/_toolSearch__problems__logs.json)

Steps to Reproduce:

1. In a fresh copilot session:
- model: GPT-5.6 Luna (or any GPT-5.6 model)
- tools: enable _only_ Built-In tools (no extension/mcp tools, I have 52 tools total selected)
2. Prompt: "#toolSearch *"