[kaiserls]

Summary

• Add a Python-only Pixi project fixture and use it as the VS Code extension test workspace.
• Run pixi install --locked before the VS Code test host starts so the fixture environment exists on fresh checkouts.
• Reproduce interpreter selection failure through the Python Environments API.
• Register the Pixi environment and package managers with the extension ID, which keeps their IDs aligned with discovered Pixi environments in development hosts.
• Improve extension activation logging and development launch setup.

Verification

• Before the registration fix, npm run test:vscode failed because the selected Pixi environment was undefined.
• After the fix, npm run test:vscode passed (2 tests).
• npm run typecheck and npm run lint passed.

The reproducer and fix are separate commits (e0868c1, 30b73a3).