[ota] Improve error message when device closes connection without responding (#13562)

esphome/espota2.py

@@ -154,6 +154,12 @@ def check_error(data: list[int] | bytes, expect: int | list[int] | None) -> None
     """
     if not expect:
         return
+    if not data:
+        raise OTAError(
+            "Error: Device closed connection without responding. "
+            "This may indicate the device ran out of memory, "
+            "a network issue, or the connection was interrupted."
+        )
     dat = data[0]
     if dat == RESPONSE_ERROR_MAGIC:
         raise OTAError("Error: Invalid magic byte")

tests/unit_tests/test_espota2.py

@@ -192,6 +192,20 @@ def test_check_error_unexpected_response() -> None:
         espota2.check_error([0x7F], [espota2.RESPONSE_OK, espota2.RESPONSE_AUTH_OK])
 
 
+def test_check_error_empty_data() -> None:
+    """Test check_error raises error when device closes connection without responding."""
+    with pytest.raises(
+        espota2.OTAError, match="Device closed connection without responding"
+    ):
+        espota2.check_error([], [espota2.RESPONSE_OK])
+
+    # Also test with empty bytes
+    with pytest.raises(
+        espota2.OTAError, match="Device closed connection without responding"
+    ):
+        espota2.check_error(b"", [espota2.RESPONSE_OK])
+
+
 def test_send_check_with_various_data_types(mock_socket: Mock) -> None:
     """Test send_check handles different data types."""