Prevent double/triple activation from two extensions

[anthonykim1]

Resolves: microsoft/vscode-python-environments#1325

Cause: The Python extension only backed off if the user explicitly opted into the envs extension (useEnvironmentsExtension: true). But the envs extension activates by default whenever it's installed — it only backs off if the setting is explicitly false. So in this case, (extension installed, setting not touched), both fired.

Added shouldEnvExtHandleActivation() — a function that mirrors the envs extension's own logic: "am I installed and not explicitly disabled?" Used it in all 3 places the Python extension triggers terminal activation to bail out when the envs extension will handle it. This covers global, workspace, and folder-level settings.

We want exactly one extension activates the terminal, never both, never neither.