fix: streamline environment retrieval in venvManager and pyenvManager

[karthiknadig]

Fixes #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:

// 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.