[web_server_idf] Build state documents in a stack arena so a state event does no heap allocation (#19383)

tests/components/json/__init__.py

@@ -1,3 +1,6 @@
+import functools
+
+from esphome.components.json import enable_arena
 from tests.testing_helpers import ComponentManifestOverride
 
 
@@ -7,3 +10,12 @@ def override_manifest(manifest: ComponentManifestOverride) -> None:
     # library registration to happen, otherwise json_util.cpp fails to find
     # ArduinoJson.h.
     manifest.enable_codegen()
+    # The JsonArena host test needs the arena compiled in, as a consumer would request it
+    real_to_code = manifest.to_code
+
+    @functools.wraps(real_to_code)
+    async def to_code_with_arena(config):
+        await real_to_code(config)
+        enable_arena()
+
+    manifest.to_code = to_code_with_arena