Fix REST sensor charset handling to respect Content-Type header (#148223)

tests/test_util/aiohttp.py

@@ -191,7 +191,6 @@ class AiohttpClientMockResponse:
         if response is None:
             response = b""
 
-        self.charset = "utf-8"
         self.method = method
         self._url = url
         self.status = status
@@ -261,16 +260,32 @@ class AiohttpClientMockResponse:
         """Return content."""
         return mock_stream(self.response)
 
+    @property
+    def charset(self):
+        """Return charset from Content-Type header."""
+        if (content_type := self._headers.get("content-type")) is None:
+            return None
+        content_type = content_type.lower()
+        if "charset=" in content_type:
+            return content_type.split("charset=")[1].split(";")[0].strip()
+        return None
+
     async def read(self):
         """Return mock response."""
         return self.response
 
-    async def text(self, encoding="utf-8", errors="strict"):
+    async def text(self, encoding=None, errors="strict") -> str:
         """Return mock response as a string."""
+        # Match real aiohttp behavior: encoding=None means auto-detect
+        if encoding is None:
+            encoding = self.charset or "utf-8"
         return self.response.decode(encoding, errors=errors)
 
-    async def json(self, encoding="utf-8", content_type=None, loads=json_loads):
+    async def json(self, encoding=None, content_type=None, loads=json_loads) -> Any:
         """Return mock response as a json."""
+        # Match real aiohttp behavior: encoding=None means auto-detect
+        if encoding is None:
+            encoding = self.charset or "utf-8"
         return loads(self.response.decode(encoding))
 
     def release(self):