Task #139 — Modbus connection-layer config knobs (keep-alive / idle / reconnect)

Promotes the previously hardcoded transport-layer settings to ModbusDriverOptions
so users can tune them through DriverConfig JSON without recompiling.

Three new option groups:

1. KeepAlive (ModbusKeepAliveOptions): Enabled / Time / Interval / RetryCount.
   Defaults preserve the historical PR 53 wire output exactly (Enabled=true,
   Time=30s, Interval=10s, RetryCount=3). Set Enabled=false for PLCs that
   reject SO_KEEPALIVE.

2. IdleDisconnectTimeout (TimeSpan?): when set, the transport tracks last-PDU-
   success and proactively closes + reconnects on the next request after the
   threshold. Defends against silent NAT / firewall socket reaping. Default
   null = disabled (no behaviour change).

3. Reconnect (ModbusReconnectOptions): InitialDelay / MaxDelay /
   BackoffMultiplier for the post-drop reconnect loop. Defaults
   (InitialDelay=0, MaxDelay=30s, Multiplier=2.0) preserve the historical
   immediate-retry behaviour for the first attempt and add geometric backoff
   only if the reconnect itself fails. Capped at 10 attempts before propagating.

ModbusTcpTransport ctor extended with optional keepAlive / idleDisconnect /
reconnect parameters; existing 4-arg call sites continue to compile. Factory
DTO gains parallel KeepAlive / IdleDisconnectMs / Reconnect fields with
default-aware binding.

5 new ModbusConnectionOptionsTests covering the default-preservation contract
(every default field matches pre-#139) and the JSON-binding round-trip for
each knob group. Existing 204 unit tests still green.

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs

@@ -47,7 +47,11 @@ public sealed class ModbusDriver
         _options = options;
         _driverInstanceId = driverInstanceId;
         _transportFactory = transportFactory
-            ?? (o => new ModbusTcpTransport(o.Host, o.Port, o.Timeout, o.AutoReconnect));
+            ?? (o => new ModbusTcpTransport(
+                o.Host, o.Port, o.Timeout, o.AutoReconnect,
+                keepAlive: o.KeepAlive,
+                idleDisconnect: o.IdleDisconnectTimeout,
+                reconnect: o.Reconnect));
         _poll = new PollGroupEngine(
             reader: ReadAsync,
             onChange: (handle, tagRef, snapshot) =>