host: implement VirtualizationWithContext for AIX (WPAR/LPAR)

[Dylan-M]

⚠️ Review order: This PR depends on #2040 being merged first. See #2072 for the full review order.

Summary

Replaces the ErrNotImplementedError stub for VirtualizationWithContext on AIX with a real implementation that detects:

1. WPAR (Workload Partition): Checks uname -W. Returns ("wpar", "guest") if the WPAR ID is non-zero.
2. LPAR (Logical Partition via PowerVM): Checks uname -L. Returns ("powervm", "guest") if the LPAR name is not "NULL".
3. Bare metal: Returns ("", "") if neither virtualization layer is detected.

Includes unit tests covering LPAR, WPAR, bare metal, and live system scenarios.

Note: This PR depends on #2040 being merged first.

Only affects AIX — no cross-platform changes.

[Copilot]

Pull request overview

Implements AIX-specific virtualization detection in host.VirtualizationWithContext, enabling the host package to report WPAR/LPAR guest environments instead of returning ErrNotImplementedError.

Changes:

• Add AIX VirtualizationWithContext implementation detecting WPAR (uname -W) and PowerVM LPAR (uname -L), otherwise returning empty values.
• Introduce AIX command invoker dependency injection (testInvoker/getInvoker) and update AIX host functions to use it.
• Add/expand AIX unit tests, including virtualization scenarios and additional host function parsing behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
host/host_aix.go	Implements AIX virtualization detection and routes command execution through an injectable invoker; adds who parsing + timestamp helper for Users.
host/host_aix_test.go	Adds mock invoker and unit tests for virtualization (LPAR/WPAR/bare metal) and other AIX host functions.

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