Fix path-only Python project manager defaults (#1667)

## Summary

Fixes #1666.

Path-only entries are valid according to the
`python-envs.pythonProjects` setting schema:

```json
{
    "python-envs.pythonProjects": [
        {
            "path": "backend"
        }
    ]
}
```

However, the manager-setting helpers called `.length` on the omitted
`envManager` and `packageManager` properties. This caused repeated
`TypeError` failures during post-initialization telemetry and
environment lookup.

## Fix

- Treat omitted or empty project-level manager values as requests to use
the corresponding default setting.
- Mark `PythonProjectSettings.envManager` and `.packageManager` optional
so the internal type matches the contributed JSON schema.
- Add a unit test covering a path-only nested project entry and both
default-manager fallbacks.

## Validation

- Reproduced with VS Code 1.130.0 and the latest stable extensions in an
empty `--user-data-dir` / `--extensions-dir` profile.
- Installed the patched 1.37.0 VSIX in a second empty profile and
verified that the same workspace starts without `TypeError` or
post-initialization failure.
- `npm run compile-tests`
- `npm run lint`
- `npm run unittest` — 1,429 passing, 6 pending

AI disclosure: Prepared with assistance from OpenAI Codex.

Co-authored-by: Eduardo Villalpando Mello <eduardovil@microsoft.com>

Author: atomiechen

src/features/projectManager.ts

@@ -37,8 +37,8 @@ export interface PythonProjectManager extends Disposable {
 
 export interface PythonProjectSettings {
     path: string;
-    envManager: string;
-    packageManager: string;
+    envManager?: string;
+    packageManager?: string;
     workspace?: string;
     _inlineScriptRegistration?: InlineScriptProjectRegistrationMarker;
 }

src/features/settings/settingHelpers.ts

@@ -99,7 +99,7 @@ export function isDefaultEnvManagerBroken(): boolean {
 export function getDefaultEnvManagerSetting(wm: PythonProjectManager, scope?: Uri): string {
     const config = workspaceApis.getConfiguration('python-envs', scope);
     const settings = getSettings(wm, config, scope);
-    if (settings && settings.envManager.length > 0) {
+    if (settings?.envManager) {
         return settings.envManager;
     }
     // Only show the warning once per session
@@ -127,7 +127,7 @@ export function getDefaultPkgManagerSetting(
     const config = workspaceApis.getConfiguration('python-envs', scope);
 
     const settings = getSettings(wm, config, scope);
-    if (settings && settings.packageManager.length > 0) {
+    if (settings?.packageManager) {
         return settings.packageManager;
     }
 

src/test/features/settings/settingHelpers.unit.test.ts

@@ -9,6 +9,8 @@ import * as sender from '../../../common/telemetry/sender';
 import * as workspaceApis from '../../../common/workspace.apis';
 import {
     addPythonProjectSetting,
+    getDefaultEnvManagerSetting,
+    getDefaultPkgManagerSetting,
     getExactPythonProjectSetting,
     migrateGlobalDefaultEnvManagerSetting,
     registerInlineScriptPythonProjectSetting,
@@ -35,6 +37,65 @@ function getTestWorkspacePath(): string {
     return process.platform === 'win32' ? 'C:\\workspace' : '/workspace';
 }
 
+suite('Setting Helpers - Optional Project Managers', () => {
+    const VENV_MANAGER_ID = 'ms-python.python:venv';
+    const PIP_MANAGER_ID = 'ms-python.python:pip';
+    const workspacePath = getTestWorkspacePath();
+    const workspaceUri = Uri.file(workspacePath);
+    const projectUri = Uri.joinPath(workspaceUri, 'backend');
+    const workspaceFolder: WorkspaceFolder = {
+        uri: workspaceUri,
+        name: 'workspace',
+        index: 0,
+    };
+    const project = {
+        name: 'backend',
+        uri: projectUri,
+    };
+    const projectManager = {
+        get: (uri: Uri) => (uri.fsPath === projectUri.fsPath ? project : undefined),
+    } as unknown as PythonProjectManager;
+
+    teardown(() => {
+        sinon.restore();
+    });
+
+    function createProjectConfig(projectSettings: PythonProjectSettings): MockWorkspaceConfiguration {
+        const mockConfig = new MockWorkspaceConfiguration();
+        (mockConfig as any).get = <T>(key: string, defaultValue?: T): T | undefined => {
+            if (key === 'pythonProjects') {
+                return [projectSettings] as T;
+            }
+            if (key === 'defaultEnvManager') {
+                return VENV_MANAGER_ID as T;
+            }
+            if (key === 'defaultPackageManager') {
+                return PIP_MANAGER_ID as T;
+            }
+            return defaultValue;
+        };
+        return mockConfig;
+    }
+
+    test('uses default managers when a pythonProjects entry only specifies path', () => {
+        sinon.stub(workspaceApis, 'getConfiguration').returns(createProjectConfig({ path: 'backend' }));
+        sinon.stub(workspaceApis, 'getWorkspaceFolder').returns(workspaceFolder);
+
+        assert.strictEqual(getDefaultEnvManagerSetting(projectManager, projectUri), VENV_MANAGER_ID);
+        assert.strictEqual(getDefaultPkgManagerSetting(projectManager, projectUri), PIP_MANAGER_ID);
+    });
+
+    test('uses default managers when a pythonProjects entry has empty manager values', () => {
+        sinon.stub(workspaceApis, 'getConfiguration').returns(
+            createProjectConfig({ path: 'backend', envManager: '', packageManager: '' }),
+        );
+        sinon.stub(workspaceApis, 'getWorkspaceFolder').returns(workspaceFolder);
+
+        assert.strictEqual(getDefaultEnvManagerSetting(projectManager, projectUri), VENV_MANAGER_ID);
+        assert.strictEqual(getDefaultPkgManagerSetting(projectManager, projectUri), PIP_MANAGER_ID);
+    });
+});
+
 /**
  * These tests verify that manager edits without a project do not write settings
  * and are logged explicitly as ignored global edits.