Fix #1599: onDidChangePythonProjects never fires on runtime project a…

[mohityadav8]

Summary

Fixes #1599 — PythonEnvironmentApi.onDidChangePythonProjects never fired when projects were added or removed at runtime. The internal project list updated correctly, but the public event was never forwarded, so consumers (notably Pylance's per-folder interpreter support) never got notified and required a restart to pick up new projects.

Root cause

PythonEnvironmentApiImpl's constructor subscribed to environment-change events but never subscribed toprojectManager.onDidChangeProjects, so _onDidChangePythonProjects.fire(...) was never called.

Fix

• Added a previousProjects field on PythonEnvironmentApiImpl that tracks the last-known project set.
• Subscribed to projectManager.onDidChangeProjects in the constructor. On each internal change, compute the added/removed delta by diffing project URIs against the previous snapshot, update the snapshot, and fire _onDidChangePythonProjects with { added, removed } when the set actually changed.

Diffing by URI (rather than relying on the manager's internal eventpayload) was needed because the manager currently emits the full projectarray rather than a delta, and DidChangePythonProjectsEventArgs expectsadded/removed.

Testing

• npm run compile — clean build.
• Manually verified via getPythonProjects() that the set updates as expected; confirmed the new subscription fires on addPythonProject() and removePythonProject() calls.