ci: use windows-x64 pool for Windows tests (#5756)

## Summary
- Use the pre-warmed `windows-x64` stopped pool for Windows tests
- Reduces boot time from 2-3 minutes (cold-start) to ~20-30 seconds

Author: joseph-isaacs

.github/runs-on.yml

@@ -9,20 +9,3 @@ images:
     arch: "arm64"
     name: "vortex-ci-*"
     owner: "375504701696"
-
-pools:
-  # Windows pool - covers UK (8am-6pm GMT) and US-East (8am-6pm EST)
-  # Combined in UK timezone: 8am-11pm (UK 8-6 + NYC 8-6 = UK 8-11pm)
-  # Stopped instances only (~20-30s boot vs 2-3min cold-start)
-  # Cost: ~$5/month (2 × $2.40 EBS storage)
-  windows-x64:
-    runner: family=m7i/cpu=8/image=windows22-full-x64/tag=rust-test-windows
-    timezone: "Europe/London"
-    schedule:
-      - name: working-hours
-        match:
-          day: ["monday", "tuesday", "wednesday", "thursday", "friday"]
-          time: ["08:00", "23:00"]
-        stopped: 2
-      - name: default
-        stopped: 1

.github/workflows/ci.yml

@@ -493,12 +493,7 @@ jobs:
       matrix:
         include:
           - os: windows-x64
-            runner:
-              - runs-on=${{ github.run_id }}
-              - family=m7i
-              - cpu=8
-              - image=windows22-full-x64
-              - tag=rust-test-windows
+            runner: runs-on=${{ github.run_id }}/pool=windows-x64
           - os: linux-arm64
             runner:
               - runs-on=${{ github.run_id }}