Path-only pythonProjects entries crash manager initialization

@eleanorjboyd がすでに取り組んでいます。

2026年8月5日 から。

## Environment data

- Python Environments extension version: 1.36.0 (latest Marketplace stable); also reproduced from `main` at `f61acdf535b9c8e8983e6b600048f3f4de1425e1` (package version 1.37.0)
- Python extension (`ms-python.python`) version: 2026.4.0
- VS Code version (Help → About): 1.130.0 (`1b6a188127eeaf9194f945eb6eb89a657e93c54c`)
- OS and version: macOS 15.3.2 arm64
- Python version: 3.9.6
- Environment manager in use: `venv`
- Shell: zsh
- Remote / container scenario: none
- Workspace type: single-folder mono-repo
- Is this a regression? Unknown

## Repro Steps

1. Start VS Code with empty `--user-data-dir` and `--extensions-dir` directories.
2. Install only `ms-python.python` and `ms-python.vscode-pylance`; this installs `ms-python.vscode-python-envs` as a dependency.
3. Open a fresh folder containing `backend/main.py` and `backend/.venv`.
4. Add the following workspace settings:

```json
{
"python.useEnvironmentsExtension": true,
"python.analysis.usePerProjectEnvironments": true,
"python-envs.pythonProjects": [
{
"path": "backend"
}
]
}
```

5. Reload the window and open `backend/main.py`.

## Expected behavior

The path-only project entry is accepted. Because `envManager` and `packageManager` are optional in the contributed setting schema, the extension uses `python-envs.defaultEnvManager` and `python-envs.defaultPackageManager`.

## Actual behavior

Environment-manager initialization repeatedly throws `TypeError: Cannot read properties of undefined (reading 'length')`. Post-initialization telemetry and subsequent environment lookups fail, preventing reliable project environment resolution.

`getDefaultEnvManagerSetting()` and `getDefaultPkgManagerSetting()` dereference optional object properties without checking whether they exist:

```typescript
if (settings && settings.envManager.length > 0) {
// ...
if (settings && settings.packageManager.length > 0) {
```

Explicitly adding both manager properties avoids the exception, but duplicates defaults that the setting schema already defines and should not be required.

## Logs

```text
Python-envs extension version: 1.36.0
Post-initialization tasks failed: TypeError: Cannot read properties of undefined (reading 'length')
at getDefaultEnvManagerSetting (.../ms-python.vscode-python-envs-1.36.0-darwin-arm64/dist/extension.js:2:389988)
at sendManagerSelectionTelemetry (.../dist/extension.js:2:286259)

TypeError: Cannot read properties of undefined (reading 'length')
at getDefaultEnvManagerSetting (.../ms-python.vscode-python-envs-1.36.0-darwin-arm64/dist/extension.js:2:389988)
at waitForEnvManager (.../dist/extension.js:2:313554)
at async getEnvironment (.../dist/extension.js:2:384509)
```

## Additional context

The reproduction used a genuinely empty VS Code profile with no third-party extensions or pre-existing user settings.

A local fix that treats omitted manager properties as fallback-to-default values removes the runtime errors in a second fresh profile. It also passes the complete unit-test suite (1,429 passing) and lint.

AI disclosure: Prepared with assistance from OpenAI Codex.