fix: streamline environment retrieval in venvManager and pyenvManager (microsoft#1157)

Fixes microsoft#610

## Summary

This PR enables pyenv-installed Python versions to appear as available
base interpreters when creating new venv environments.

## Changes

### 1. Fixed bug in `PyEnvManager.getEnvironments()`

The filter callback for the `'global'` scope was missing a return
statement - it used braces without returning a value, causing it to
always return an empty array:

```typescript
// Before (buggy):
return this.collection.filter((env) => {
    env.group === PYENV_VERSIONS;  // Statement - doesn't return anything
});

// After (fixed):
return this.collection.filter((env) => env.group === PYENV_VERSIONS);
```

### 2. Updated `VenvManager` to use API for aggregating global
environments

Changed from using only `baseManager.getEnvironments('global')` (which
only returns system Python) to `api.getEnvironments('global')` which
aggregates environments from ALL registered managers including pyenv,
conda, etc.

This change was applied to three locations in `venvManager.ts`:
- `create()` method - when showing available base interpreters for venv
creation
- `resetGlobalEnv()` method - when resetting the global environment
reference
- `loadEnvMap()` method - when loading environment mappings

## Result

When users create a new venv environment, pyenv-installed Python
versions (those in the `PYENV_VERSIONS` group) will now appear alongside
system Python installations as available base interpreters.

Author: karthiknadig

src/managers/builtin/venvManager.ts

@@ -142,7 +142,7 @@ export class VenvManager implements EnvironmentManager {
 
             const venvRoot: Uri = Uri.file(await findParentIfFile(uri.fsPath));
 
-            const globals = await this.baseManager.getEnvironments('global');
+            const globals = await this.api.getEnvironments('global');
             let result: CreateEnvironmentResult | undefined = undefined;
             if (options?.quickCreate) {
                 // error on missing information

src/managers/pyenv/pyenvManager.ts

@@ -96,9 +96,7 @@ export class PyEnvManager implements EnvironmentManager, Disposable {
         }
 
         if (scope === 'global') {
-            return this.collection.filter((env) => {
-                env.group === PYENV_VERSIONS;
-            });
+            return this.collection.filter((env) => env.group === PYENV_VERSIONS);
         }
 
         if (scope instanceof Uri) {