G4: F04 + NEW-A — symmetric send() failure signaling across all transports

Why: TCP was the only transport that fired _disconnect_callback on a dead
transport or send failure. Serial and BLE returned silently, leaving
ConnectionManager unaware the link was down. Additionally, transport.write()
(TCP/serial) and write_gatt_char (BLE) had no try/except — any write-time
exception escaped the send() coroutine unhandled. This commit makes all three
transports symmetric: (a) fire _disconnect_callback when transport is None/dead,
(b) wrap the write call in try/except and fire _disconnect_callback on failure.
Refs: Forensics report findings F04, NEW-A

src/meshcore/ble_cx.py

@@ -171,11 +171,19 @@ class BLEConnection:
     async def send(self, data):
         if not self.client:
             logger.error("Client is not connected")
+            if self._disconnect_callback:
+                await self._disconnect_callback("ble_transport_lost")
             return False
         if not self.rx_char:
             logger.error("RX characteristic not found")
             return False
-        await self.client.write_gatt_char(self.rx_char, bytes(data), response=True)
+        try:
+            await self.client.write_gatt_char(self.rx_char, bytes(data), response=True)
+        except Exception as exc:
+            logger.warning(f"BLE write failed: {exc}")
+            if self._disconnect_callback:
+                await self._disconnect_callback(f"ble_write_failed: {exc}")
+            return False
 
     async def disconnect(self):
         """Disconnect from the BLE device."""

src/meshcore/serial_cx.py

@@ -125,11 +125,18 @@ class SerialConnection:
     async def send(self, data):
         if not self.transport:
             logger.error("Transport not connected, cannot send data")
+            if self._disconnect_callback:
+                await self._disconnect_callback("serial_transport_lost")
             return
         size = len(data)
         pkt = b"\x3c" + size.to_bytes(2, byteorder="little") + data
         logger.debug(f"sending pkt : {pkt}")
-        self.transport.write(pkt)
+        try:
+            self.transport.write(pkt)
+        except OSError as exc:
+            logger.warning(f"Serial write failed: {exc}")
+            if self._disconnect_callback:
+                await self._disconnect_callback(f"serial_write_failed: {exc}")
 
     async def disconnect(self):
         """Close the serial connection."""

src/meshcore/tcp_cx.py

@@ -137,7 +137,12 @@ class TCPConnection:
         size = len(data)
         pkt = b"\x3c" + size.to_bytes(2, byteorder="little") + data
         logger.debug(f"sending pkt : {pkt}")
-        self.transport.write(pkt)
+        try:
+            self.transport.write(pkt)
+        except (OSError, ConnectionResetError) as exc:
+            logger.warning(f"TCP write failed: {exc}")
+            if self._disconnect_callback:
+                await self._disconnect_callback(f"tcp_write_failed: {exc}")
 
     async def disconnect(self):
         """Close the TCP connection."""