Split macOS Intel bundler tests across four tag-group runners

A single macos-15-intel runner took around 40 minutes. Reuse the
Windows tag groups to fan the run out to four runners, keeping each
under roughly ten minutes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hsbt

.github/workflows/bundler.yml

@@ -40,8 +40,12 @@ jobs:
           - { os: { name: macOS, value: macos-26 }, ruby: { name: ruby-3.4, value: 3.4.9 }, timeout: 90 }
           - { os: { name: macOS, value: macos-26 }, ruby: { name: ruby-4.0, value: 4.0.5 }, timeout: 90 }
 
-          # Regression coverage for the Intel architecture with the latest Ruby only
-          - { os: { name: macOS-intel, value: macos-15-intel }, ruby: { name: ruby-4.0, value: 4.0.5 }, timeout: 90 }
+          # Regression coverage for the Intel architecture with the latest Ruby only.
+          # Split across runners using the Windows tag groups to keep each run reasonable.
+          - { os: { name: macOS-intel, value: macos-15-intel }, ruby: { name: ruby-4.0, value: 4.0.5 }, timeout: 90, group: a }
+          - { os: { name: macOS-intel, value: macos-15-intel }, ruby: { name: ruby-4.0, value: 4.0.5 }, timeout: 90, group: b }
+          - { os: { name: macOS-intel, value: macos-15-intel }, ruby: { name: ruby-4.0, value: 4.0.5 }, timeout: 90, group: c }
+          - { os: { name: macOS-intel, value: macos-15-intel }, ruby: { name: ruby-4.0, value: 4.0.5 }, timeout: 90, group: d }
 
           # Ruby 3.2 is about 20 minutes slower than 3.3/3.4, so it will be excluded from testing.
           - { os: { name: Windows, value: windows-2025 }, ruby: { name: ruby-3.3, value: 3.3.11 }, timeout: 150, group: a }
@@ -129,11 +133,15 @@ jobs:
       - name: Run Test (CRuby)
         run: |
           bin/parallel_rspec
-        if: matrix.ruby.name != 'jruby' && matrix.ruby.name != 'truffleruby' && matrix.os.name != 'Windows'
+        if: matrix.ruby.name != 'jruby' && matrix.ruby.name != 'truffleruby' && matrix.os.name != 'Windows' && matrix.os.name != 'macOS-intel'
       - name: Run Test (CRuby on Windows - Group ${{ matrix.group }})
         run: |
           bin/parallel_rspec --tag windows_${{ matrix.group }}
         if: matrix.ruby.name != 'jruby' && matrix.ruby.name != 'truffleruby' && matrix.os.name == 'Windows'
+      - name: Run Test (CRuby on macOS Intel - Group ${{ matrix.group }})
+        run: |
+          bin/parallel_rspec --tag windows_${{ matrix.group }}
+        if: matrix.ruby.name != 'jruby' && matrix.ruby.name != 'truffleruby' && matrix.os.name == 'macOS-intel'
       - name: Run Test (JRuby)
         run: |
           bin/parallel_rspec --tag jruby_only --tag jruby