Startup stalls and Python environments appear missing during activation

@eleanorjboyd y travaille déjà.

Depuis le 3/4/2026.

## Problem
When VS Code opens a Python workspace, the extension can stall during startup and some environment managers fail to register. Users then see missing or incorrect Python environments until reload.

## What users might see

- Slow startup or temporary UI freeze when opening a project
- High CPU during startup
- Environment list missing expected entries (especially non-system managers)
- Environment selection/activation not working right away
- In some cases, repeated terminal/process churn

## How it arises

- On activation, multiple managers register in parallel.
- Some managers fall back to PET-based detection during registration.
- If PET is still cold/initializing (common on slower, remote, or containerized setups), requests can timeout.
- Timeout/retry behavior can cascade, leaving managers unregistered for that session.

## Steps to reproduce

- Open a Python project in a fresh VS Code session.
- Use a machine/workspace with slower startup conditions (remote, container, or heavy load).
- Wait for Python Environments extension activation.
- Open the environment picker/tree.
- Observe startup delay and potentially missing managers/environments.
- Expected behavior
- Startup remains responsive and manager registration does not block or fail; environments appear reliably.

## Actual behavior

- Startup can hang briefly, and some managers/environments are unavailable until another refresh or reload.