Add package-manager integration lifecycle coverage

[edvilme]

Summary

Add readable baseline integration coverage for each registered package manager before the command refactor in #1686 lands.

Scope

• Drive tests from a package-manager fixture list with a registry completeness guard.
• Exercise stateful install/list/direct-dependency/uninstall lifecycles in disposable environments.
• Cover Pip (normal and uv-backed when available) and Conda; explicitly defer Poetry with rationale.
• Expose live registered package managers through a narrowly test-only integration seam so non-public capabilities can be covered.
• Restore settings and remove all test-owned projects/environments/directories reliably.

Related: #1622, #1686