release: Bump extension version to 2026.6.0 (#26066)

## Summary
- bump the extension version from `2026.5.0-dev` to `2026.6.0`
- refresh `package-lock.json` with `npm install`

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: eleanorjboyd

package-lock.json

@@ -2,7 +2,7 @@
     "name": "python",
     "displayName": "Python",
     "description": "Python language support with extension access points for IntelliSense (Pylance), Debugging (Python Debugger), linting, formatting, refactoring, unit tests, and more.",
-    "version": "2026.5.0-dev",
+    "version": "2026.6.0",
     "featureFlags": {
         "usingNewInterpreterStorage": true
     },