CI: switch Python setup to astral-sh/setup-uv, pin macOS to 15

Replace actions/setup-python with astral-sh/setup-uv@v7, mirroring the
upstream pythonnet workflow. Uses the cpython-<version><suffix> format for
architecture-specific Python builds, and enables uv caching.

Pin macOS runner to macos-15 instead of macos-latest.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jhonabreul

.github/workflows/main.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   build-test:
     name: Build and Test
-    runs-on: ${{ matrix.os }}-latest
+    runs-on: ${{ matrix.os == 'macos' && 'macos-15' || format('{0}-latest', matrix.os) }}
     timeout-minutes: 15
 
     strategy:
@@ -34,10 +34,12 @@ jobs:
           dotnet-version: '10.0.x'
 
       - name: Set up Python ${{ matrix.python }}
-        uses: actions/setup-python@v5
+        uses: astral-sh/setup-uv@v7
         with:
-          python-version: ${{ matrix.python }}
-          architecture: ${{ matrix.platform }}
+          python-version: cpython-${{ matrix.python }}${{ matrix.os == 'windows' && matrix.platform == 'x86' && '-windows-x86-none' || matrix.os == 'windows' && matrix.platform == 'x64' && '-windows-x86_64-none' || matrix.os == 'macos' && matrix.platform == 'x64' && '-macos-x86_64-none' || '' }}
+          cache-python: true
+          activate-environment: true
+          enable-cache: true
 
       - name: Install dependencies
         run: |