Add package manager command smoke tests

[edvilme]

Summary

Adds smoke unit coverage for every concrete package manager command introduced in #1621.

Scope

• Covers all 12 Pip/UV command classes.
• Covers all 5 Conda command classes.
• Covers all 5 Poetry command classes.
• Mocks external command execution with minimal successful output.
• Verifies only that each command executes without throwing or rejecting.

These tests intentionally do not assert command arguments, parsed values, cancellation, or failure behavior.

PR stack

1. Introduce package manager command classes #1621: command classes and implementations.
2. This PR: command smoke tests.
3. Adopt package manager command classes #1678: adoption and cleanup.

Testing

• npm run lint
• npm run compile-tests
• npm run unittest (1,470 passing, 4 pending)

[edvilme]

Contributes to #1622