azd tool: improve VS Code extension visibility when VS Code is not installed

## Problem

When VS Code is not installed, `azd tool list` shows VS Code extensions as "Not Installed" (same status as CLI tools that haven't been installed yet), and `azd tool install` fails with a package-manager-unavailable error. This is confusing because the user can't install these tools even if they wanted to.

## Options

1. **Hide extensions** whose prerequisite (VS Code) isn't available
2. **Show distinct status** like "Unavailable (requires VS Code)" instead of "Not Installed"
3. **Add `--category` filter flag** so users can scope the list (e.g., `azd tool list --category cli`)

## Context

Raised by @vhvb1989 in PR #7450 review.

## Related

- PR #7450
- Issue #7845 (Epic A: Core Command Group)