dbt.dbtIntegration: "fusion" fails detection when dbt Fusion is installed as pip package (no Python module)

**Environment**

- Extension version: 0.61.7
- dbt Fusion: 2.0.0-preview.190
- OS: macOS

**Steps to reproduce**

1. Install dbt Fusion via pip into a venv ([pip install](https://docs.getdbt.com/docs/local/install-dbt?version=2.0&name=Fusion#pip-installation-for-windows-macos-and-linux))
2. Set "dbt.dbtIntegration": "fusion" in .vscode/settings.json
3. Open the project in VS Code
4. Detect from terminal results in
```
python3 -c 'import sys; __import__("dbt"); print("__DBT_DETECT_START__"); print(
sys.executable); print("__DBT_DETECT_END__")'
Traceback (most recent call last):
File "", line 1, in
import sys; __import__("dbt"); print("__DBT_DETECT_START__"); print(sys.executable); print("__DBT_DETECT_END__")
~~~~~~~~~~^^^^^^^
ModuleNotFoundError: No module named 'dbt'
```

**Expected:** Extension detects dbt Fusion binary and initializes successfully

Actual: Extension shows "dbt fusion is not installed" because it runs python3 -c 'import dbt' — which fails since the Fusion pip package installs a binary only, with no importable Python module

Root cause: In fusion mode, the extension should detect the dbt binary directly (e.g. dbt --version) rather than attempting a Python import