[core] Leave the precompiled header off on Windows for a GCC with bug 14940 (#19875)

tests/unit_tests/conftest.py

@@ -178,6 +178,15 @@ def held_lock() -> Callable[..., Callable[..., None]]:
 
 @pytest.fixture(autouse=True)
 def _default_pch_knobs(monkeypatch: pytest.MonkeyPatch) -> None:
-    """The precompiled header knobs change what the generators emit; a value
-    exported in the developer's or CI's shell must not reach the tests."""
-    monkeypatch.delenv("ESPHOME_PCH_ENABLE", raising=False)
+    """The knob changes what the generators emit: no ambient value, and on,
+    so the Windows runner asks no compiler."""
+    monkeypatch.setenv("ESPHOME_PCH_ENABLE", "1")
+
+
+@pytest.fixture
+def windows_gcc_rule(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Windows host, knob unset: the compiler version decides."""
+    from esphome.build_helpers import pch
+
+    monkeypatch.delenv("ESPHOME_PCH_ENABLE")
+    monkeypatch.setattr(pch.sys, "platform", "win32")