[smart_holder] Configure only one job per OS kind to build and run te…

…sts with `-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT` (#5307)

* Configure only one job per OS kind to build and run tests with `-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT`

Remove ci_sh_def.yml

Piggy-backed: Add `smart_holder` to `branches` in emscripten.yaml

* Fix misunderstanding.

* Add `/GR /EHsc` for Windows.

* #error "Intentional Breakage" (to verify: PYBIND11_ACTUALLY_USING_SMART_HOLDER_AS_DEFAULT)

* Revert "#error "Intentional Breakage" (to verify: PYBIND11_ACTUALLY_USING_SMART_HOLDER_AS_DEFAULT)"

This reverts commit 4f6f5cb.

.github/workflows/ci.yml

@@ -68,7 +68,18 @@ jobs:
           # Extra ubuntu latest job
           - runs-on: ubuntu-latest
             python: '3.11'
-
+          - runs-on: ubuntu-latest
+            python: '3.12'
+            args: >
+              -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+          - runs-on: macos-13
+            python: '3.12'
+            args: >
+              -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+          - runs-on: windows-2022
+            python: '3.12'
+            args: >
+              -DCMAKE_CXX_FLAGS="/DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT /GR /EHsc"
 
     name: "🐍 ${{ matrix.python }} • ${{ matrix.runs-on }} • x64 ${{ matrix.args }}"
     runs-on: ${{ matrix.runs-on }}