test: add test for network errors (httpx.ConnectError)

The docstring of fetch_btc_eur_price mentions it raises httpx.RequestError
on network errors, but this wasn't tested. Add test for ConnectError.

backend/tests/test_price_history.py

@@ -65,6 +65,22 @@ class TestFetchBtcEurPrice:
         ):
             await fetch_btc_eur_price()
 
+    @pytest.mark.asyncio
+    async def test_raises_on_network_error(self):
+        """Verify network errors (ConnectError, TimeoutException) are propagated."""
+        import httpx
+
+        mock_client = AsyncMock()
+        mock_client.get.side_effect = httpx.ConnectError("Connection refused")
+        mock_client.__aenter__.return_value = mock_client
+        mock_client.__aexit__.return_value = None
+
+        with (
+            patch("price_fetcher.httpx.AsyncClient", return_value=mock_client),
+            pytest.raises(httpx.ConnectError),
+        ):
+            await fetch_btc_eur_price()
+
     @pytest.mark.asyncio
     async def test_raises_on_invalid_response_format(self):
         """Verify ValueError is raised for unexpected response format."""