fix(modbus): transport self-heals with one throttled in-gate connect; PDU length check

P3: a torn-down socket is re-dialled inside the gate (one attempt per backoff
window) instead of failing fast with 'Transport not connected' until the 90 s
comms-loss watchdog re-initialised the driver.
L3: ConnectAsync disposes the previous/failed TcpClient, DNS runs under the
per-op timeout, and the reconnect never sleeps inside the gate (the old
10-attempt backoff held it for ~81 s).
H9: an MBAP response with no PDU, or an exception PDU with no code, is a
TruncatedFrame desync rather than an IndexOutOfRangeException.

Refs #741
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusTransportUnavailableException.cs

@@ -0,0 +1,23 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+/// <summary>
+///     Raised when a Modbus transport has no live socket and cannot get one for this call: the one
+///     in-gate connect attempt failed, the reconnect is inside its backoff window, or auto-reconnect is
+///     disabled. It says the ENDPOINT is unreachable, so the driver fails the rest of the batch fast
+///     instead of paying the same failure once per tag.
+/// </summary>
+/// <remarks>
+///     Derives from <see cref="IOException"/> so every existing socket-level classifier
+///     (<c>ModbusSocketLifecycle.IsSocketLevelFailure</c>, the driver's comms-fault mapping) treats it as
+///     a communication failure without extra plumbing.
+/// </remarks>
+public sealed class ModbusTransportUnavailableException : IOException
+{
+    /// <summary>Initializes a new instance of the <see cref="ModbusTransportUnavailableException"/> class.</summary>
+    /// <param name="message">A human-readable description.</param>
+    /// <param name="inner">The originating exception, if any.</param>
+    public ModbusTransportUnavailableException(string message, Exception? inner = null)
+        : base(message, inner)
+    {
+    }
+}