CmdPal: Fix GPU index out of range crash in PerfMon widget (#48103)

## Summary

Fixes #47821

The GPU Performance Monitor widget crashes with
`IndexOutOfRangeException` on systems where GPU performance counters
fail to enumerate (common on Intel Arc and hybrid GPU configurations).
The dock band shows `???` and opening the flyout causes an error.

## Root Cause

`GPUStats.CreateGPUImageUrl()` accessed `_stats[index]` without bounds
checking. When `GetGPUPerfCounters()` finds no matching counter
instances, `_stats` remains empty but callers still pass index 0.

`GetGPUName()`, `GetGPUUsage()`, and `GetGPUTemperature()` already have
proper guards (`if (_stats.Count <= index) return ...`) — this fix adds
the same pattern to the one remaining unguarded method.

## Changes

- `GPUStats.cs`: Add bounds check to `CreateGPUImageUrl()` — return
empty string if index out of range

Co-authored-by: root <root@io.bbq>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>