disk: implement IOCountersWithContext for AIX nocgo via iostat

[Dylan-M]

Summary

Replaces the ErrNotImplementedError stub for IOCountersWithContext on the AIX nocgo path with a real implementation that parses iostat -d output.

Returns ReadBytes and WriteBytes for each disk by parsing the KB read/written columns and converting to bytes.

The CGO path already has a perfstat-based implementation (merged via #2016). This provides the nocgo fallback for environments where CGO is not available.

Only affects AIX nocgo path — no cross-platform changes.

[Copilot]

Pull request overview

Implements an AIX !cgo (nocgo) fallback for disk I/O counters by parsing iostat -d, replacing the previous ErrNotImplementedError stub, and refactors UsageWithContext parsing to match mountpoints more reliably while avoiding repeated mount subprocess calls.

Changes:

• Implement IOCountersWithContext for AIX nocgo by parsing iostat -d and returning per-disk ReadBytes/WriteBytes.
• Add a helper to build a device→fstype map from mount output and use it in UsageWithContext.
• Fix UsageWithContext parsing to match the Path/mountpoint column and handle - values more robustly.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[shirou]

The UsageWithContext changes (416f4ed) are already merged via #2032, so the effective diff here is only be66821 (IOCountersWithContext implementation). Reviewed that commit — looks good.