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.

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusConnectionOptionsTests.cs

@@ -0,0 +1,101 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+/// <summary>
+///     #139 connection-layer config knobs: keep-alive, idle-disconnect, reconnect backoff.
+///     Coverage focuses on default behaviour (matches pre-#139 wire output exactly) and the
+///     DTO-binding path so users can drive these from JSON without editing C#.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class ModbusConnectionOptionsTests
+{
+    [Fact]
+    public void Defaults_Match_Historical_Behaviour()
+    {
+        var opts = new ModbusDriverOptions();
+
+        opts.KeepAlive.Enabled.ShouldBeTrue();
+        opts.KeepAlive.Time.ShouldBe(TimeSpan.FromSeconds(30));
+        opts.KeepAlive.Interval.ShouldBe(TimeSpan.FromSeconds(10));
+        opts.KeepAlive.RetryCount.ShouldBe(3);
+
+        opts.IdleDisconnectTimeout.ShouldBeNull();
+
+        opts.Reconnect.InitialDelay.ShouldBe(TimeSpan.Zero);
+        opts.Reconnect.MaxDelay.ShouldBe(TimeSpan.FromSeconds(30));
+        opts.Reconnect.BackoffMultiplier.ShouldBe(2.0);
+    }
+
+    [Fact]
+    public void Factory_Reads_KeepAlive_Knobs_From_Json()
+    {
+        const string json = """
+        {
+          "host": "10.0.0.10",
+          "tags": [],
+          "keepAlive": { "enabled": false, "timeMs": 60000, "intervalMs": 5000, "retryCount": 5 }
+        }
+        """;
+        var driver = ModbusDriverFactoryExtensions.CreateInstance("modbus-1", json);
+        // Reach into options via reflection — the factory's options field is internal.
+        var opts = (ModbusDriverOptions)typeof(ModbusDriver)
+            .GetField("_options", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!
+            .GetValue(driver)!;
+
+        opts.KeepAlive.Enabled.ShouldBeFalse();
+        opts.KeepAlive.Time.ShouldBe(TimeSpan.FromMinutes(1));
+        opts.KeepAlive.Interval.ShouldBe(TimeSpan.FromSeconds(5));
+        opts.KeepAlive.RetryCount.ShouldBe(5);
+    }
+
+    [Fact]
+    public void Factory_Reads_IdleDisconnect_From_Json()
+    {
+        const string json = """{ "host": "10.0.0.10", "tags": [], "idleDisconnectMs": 120000 }""";
+        var driver = ModbusDriverFactoryExtensions.CreateInstance("modbus-1", json);
+        var opts = (ModbusDriverOptions)typeof(ModbusDriver)
+            .GetField("_options", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!
+            .GetValue(driver)!;
+
+        opts.IdleDisconnectTimeout.ShouldBe(TimeSpan.FromMinutes(2));
+    }
+
+    [Fact]
+    public void Factory_Reads_Reconnect_Backoff_From_Json()
+    {
+        const string json = """
+        {
+          "host": "10.0.0.10",
+          "tags": [],
+          "reconnect": { "initialDelayMs": 500, "maxDelayMs": 60000, "backoffMultiplier": 1.5 }
+        }
+        """;
+        var driver = ModbusDriverFactoryExtensions.CreateInstance("modbus-1", json);
+        var opts = (ModbusDriverOptions)typeof(ModbusDriver)
+            .GetField("_options", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!
+            .GetValue(driver)!;
+
+        opts.Reconnect.InitialDelay.ShouldBe(TimeSpan.FromMilliseconds(500));
+        opts.Reconnect.MaxDelay.ShouldBe(TimeSpan.FromMinutes(1));
+        opts.Reconnect.BackoffMultiplier.ShouldBe(1.5);
+    }
+
+    [Fact]
+    public void Factory_With_Empty_Json_Uses_All_Defaults()
+    {
+        const string json = """{ "host": "10.0.0.10", "tags": [] }""";
+        var driver = ModbusDriverFactoryExtensions.CreateInstance("modbus-1", json);
+        var opts = (ModbusDriverOptions)typeof(ModbusDriver)
+            .GetField("_options", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!
+            .GetValue(driver)!;
+
+        // Every connection-layer field must match the historical defaults so existing config
+        // rows stay bit-for-bit identical after #139.
+        opts.KeepAlive.Enabled.ShouldBeTrue();
+        opts.IdleDisconnectTimeout.ShouldBeNull();
+        opts.Reconnect.InitialDelay.ShouldBe(TimeSpan.Zero);
+        opts.AutoReconnect.ShouldBeTrue();
+    }
+}