Pylance startup blocked on environment enumeration (slow first-run / refresh) #26020

Problem

On first run (and during any full environment refresh), Pylance can take a very long time to start — in local testing the language server initialize was delayed by ~100+ seconds. The root cause is that startup blocks on environment work that doesn't need to gate the server:

• getActiveInterpreter() waits on the in-progress environment enumeration/refresh (which can take tens of seconds, especially with conda and many envs), so the Python extension can't hand Pylance an interpreter quickly.
• Activation awaits interpreter auto-selection rather than letting it complete in the background.
• Conda deep probes (registry scan / conda info --json) run on the startup path even when the environments extension already owns discovery.

Desired behavior

Startup should be fast even on first run. getActiveInterpreter() should return within ~100ms — serving the last-known/persisted interpreter when discovery is slow — and the real value should resolve in the background and update Pylance via the normal change notification (it's acceptable that the active interpreter may change after a full refresh).

Notes

The fix spans three repositories:

• microsoft/vscode-python (this repo) — non-blocking interpreter resolution + activation.
• microsoft/vscode-python-environments — return last-known environment when getEnvironment times out.
• the Pylance server — don't block initialize on a recursive workspace-folder walk.