Mark connection as disconnected on TCP errors

Set self.connected = False in send_data(), receive_data(), and
_recv_exact() when TCP errors or timeouts occur. This ensures
get_connected() returns False after a network failure, preventing
stale data from being returned on subsequent operations.

After a TCP error, users must reconnect to the PLC, which is the
correct behavior per the original snap7 documentation.

Fixes #70

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gijzelaerr

snap7/connection.py

@@ -129,6 +129,7 @@ def send_data(self, data: bytes) -> None:
             self.socket.sendall(tpkt_frame)
             logger.debug(f"Sent {len(tpkt_frame)} bytes")
         except socket.error as e:
+            self.connected = False
             raise S7ConnectionError(f"Send failed: {e}")
 
     def receive_data(self) -> bytes:
@@ -162,8 +163,10 @@ def receive_data(self) -> bytes:
             return self._parse_cotp_data(payload)
 
         except socket.timeout:
+            self.connected = False
             raise S7TimeoutError("Receive timeout")
         except socket.error as e:
+            self.connected = False
             raise S7ConnectionError(f"Receive failed: {e}")
 
     def _tcp_connect(self) -> None:
@@ -373,11 +376,14 @@ def _recv_exact(self, size: int) -> bytes:
             try:
                 chunk = self.socket.recv(size - len(data))
                 if not chunk:
+                    self.connected = False
                     raise S7ConnectionError("Connection closed by peer")
                 data.extend(chunk)
             except socket.timeout:
+                self.connected = False
                 raise S7TimeoutError("Receive timeout")
             except socket.error as e:
+                self.connected = False
                 raise S7ConnectionError(f"Receive error: {e}")
 
         return bytes(data)