Cover the unresolved-toolchain ordering guard

tests/unit_tests/test_config_validation.py

@@ -3183,6 +3183,17 @@ def test_require_platformio_toolchain() -> None:
         validator(config)
 
 
+def test_check_supported_toolchain_unresolved_is_an_ordering_bug() -> None:
+    """Calling the check before resolution fails naming the ordering bug,
+    not a user-facing unsupported-toolchain error."""
+    from esphome.const import Toolchain
+    from esphome.core import CORE
+
+    CORE.toolchain = None
+    with pytest.raises(Invalid, match="not resolved before RP2 validation"):
+        cv.check_supported_toolchain("RP2", (Toolchain.PLATFORMIO,))
+
+
 @pytest.mark.parametrize(
     ("platform", "minimal_config"),
     [