Fix environment extension activation scope mismatch

[atomiechen]

Fixes #26068.

Problem

useEnvExtension() used the effective value of python.useEnvironmentsExtension, while the Python Environments extension declines activation if any user, workspace, or workspace-folder scope explicitly sets it to false.

With a user-level false and workspace-level true, the Python extension therefore tried to consume an API that the Python Environments extension did not export and failed on undefined.onDidChangeEnvironment.

This change aligns the activation checks; it does not change configuration-scope precedence.

Change

• Require both the effective opt-in and shouldEnvExtHandleActivation() before using the Python Environments API.
• Add a regression test for conflicting user/workspace settings.

Validation

• TypeScript compile and targeted ESLint pass.
• Focused activation-check suite: 7 passing.
• Fresh stable baseline reproduces the exception.
• Patched Python VSIX with the same settings activates and uses the legacy locator.
• Stable extensions with the user-level false removed start Python Environments and discover the nested .venv.

AI disclosure: Prepared with assistance from OpenAI Codex.