fix: add nim libp2p echo interop

tests/interop/nim_libp2p/nim_echo_server.nim

@@ -0,0 +1,108 @@
+{.used.}
+
+import chronos
+import stew/byteutils
+import libp2p
+
+##
+# Simple Echo Protocol Implementation for py-libp2p Interop Testing
+##
+const EchoCodec = "/echo/1.0.0"
+
+type EchoProto = ref object of LPProtocol
+
+proc new(T: typedesc[EchoProto]): T =
+  proc handle(conn: Connection, proto: string) {.async: (raises: [CancelledError]).} =
+    try:
+      echo "Echo server: Received connection from ", conn.peerId
+
+      # Read and echo messages in a loop
+      while not conn.atEof:
+        try:
+          # Read length-prefixed message using nim-libp2p's readLp
+          let message = await conn.readLp(1024 * 1024) # Max 1MB
+          if message.len == 0:
+            echo "Echo server: Empty message, closing connection"
+            break
+
+          let messageStr = string.fromBytes(message)
+          echo "Echo server: Received (", message.len, " bytes): ", messageStr
+
+          # Echo back using writeLp
+          await conn.writeLp(message)
+          echo "Echo server: Echoed message back"
+
+        except CatchableError as e:
+          echo "Echo server: Error processing message: ", e.msg
+          break
+
+    except CancelledError as e:
+      echo "Echo server: Connection cancelled"
+      raise e
+    except CatchableError as e:
+      echo "Echo server: Exception in handler: ", e.msg
+    finally:
+      echo "Echo server: Connection closed"
+      await conn.close()
+
+  return T.new(codecs = @[EchoCodec], handler = handle)
+
+##
+# Create QUIC-enabled switch
+##
+proc createSwitch(ma: MultiAddress, rng: ref HmacDrbgContext): Switch =
+  var switch = SwitchBuilder
+    .new()
+    .withRng(rng)
+    .withAddress(ma)
+    .withQuicTransport()
+    .build()
+  result = switch
+
+##
+# Main server
+##
+proc main() {.async.} =
+  let
+    rng = newRng()
+    localAddr = MultiAddress.init("/ip4/0.0.0.0/udp/0/quic-v1").tryGet()
+    echoProto = EchoProto.new()
+
+  echo "=== Nim Echo Server for py-libp2p Interop ==="
+
+  # Create switch
+  let switch = createSwitch(localAddr, rng)
+  switch.mount(echoProto)
+
+  # Start server
+  await switch.start()
+
+  # Print connection info
+  echo "Peer ID: ", $switch.peerInfo.peerId
+  echo "Listening on:"
+  for addr in switch.peerInfo.addrs:
+    echo "  ", $addr, "/p2p/", $switch.peerInfo.peerId
+  echo "Protocol: ", EchoCodec
+  echo "Ready for py-libp2p connections!"
+  echo ""
+
+  # Keep running
+  try:
+    await sleepAsync(100.hours)
+  except CancelledError:
+    echo "Shutting down..."
+  finally:
+    await switch.stop()
+
+# Graceful shutdown handler
+proc signalHandler() {.noconv.} =
+  echo "\nShutdown signal received"
+  quit(0)
+
+when isMainModule:
+  setControlCHook(signalHandler)
+  try:
+    waitFor(main())
+  except CatchableError as e:
+    echo "Error: ", e.msg
+    quit(1)