Direct-download tool installs not discoverable by PATH-based detector

## Problem

Direct-download tool installs place artifacts in the azd config tools directory (`~/.azd/tools/`), but the tool detector uses PATH-based lookup. Unless that directory is on PATH (or a shim is created), direct-download installs will consistently fail post-install verification.

## Context

Raised in PR #7450 review thread by copilot-pull-request-reviewer.

## Options to Investigate

1. **Create shims** in a well-known PATH directory pointing to the installed binary
2. **Modify PATH** for the current process after direct-download install
3. **Extend detection** to also probe the config tools directory
4. **Add to PATH guidance** — print a message telling the user to add the directory to their PATH

## Related

- PR #7450 (azd tool command group)
- Issue #7845 (Epic A: Core Command Group)