[core] Clear loaded_platforms on CORE.reset() (#19268)

esphome/core/__init__.py

@@ -715,6 +715,7 @@ class EsphomeCore:
         self.defines = set()
         self.platformio_options = {}
         self.loaded_integrations = set()
+        self.loaded_platforms = set()
         self.component_ids = set()
         self.platform_counts = defaultdict(int)
         self.unique_ids = {}