fix(driver-modbus): resolve Low code-review findings (Driver.Modbus-003,007,008,009,010,011,012)

- Driver.Modbus-003: route every _health access through ReadHealth /
  WriteHealth helpers backed by Volatile.Read / Volatile.Write so a
  burst of concurrent ReadAsync callers always sees a complete snapshot.
- Driver.Modbus-007: promoted the Int64 / UInt64 → Int32 surfacing
  caveat to a full <remarks> block; rewrote DisableFC23's doc to flag it
  as reserved / no-op.
- Driver.Modbus-008: deleted stale duplicate doc, rewrote the
  prohibition-block summaries to credit the shipped re-probe loop, and
  removed the unused 'status' local in the ModbusException catch arm.
- Driver.Modbus-009: bind-time validation rejects StringLength < 1 for
  String tags; ModbusTcpTransport clamps keep-alive intervals to whole
  seconds (>=1).
- Driver.Modbus-010: documented WriteOnChangeOnly's cache-invalidation
  policy (reads-only) and the write-only-tag caveat.
- Driver.Modbus-011: collected the scattered instance fields into a
  single contiguous block at the top of ModbusDriver.
- Driver.Modbus-012: covered the previously-uncovered Reinitialize
  state-hygiene, malformed/truncated/empty-bitmap response, and
  DisposeAsync teardown paths.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -91,13 +91,31 @@ public sealed class ModbusTcpTransport : IModbusTransport
         try
         {
             client.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
-            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime, (int)opts.Time.TotalSeconds);
-            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval, (int)opts.Interval.TotalSeconds);
+            // Driver.Modbus-009: a TimeSpan < 1s previously truncated to 0 via the int cast,
+            // which Windows / Linux interpret as "use the default" — silently defeating the
+            // configured keep-alive timing. Round up to at least 1 second so a sub-second
+            // configuration still produces a real keep-alive cadence. Negative values are
+            // also clamped to 1 to avoid surfacing as OS errors.
+            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime,
+                ClampToWholeSeconds(opts.Time));
+            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval,
+                ClampToWholeSeconds(opts.Interval));
             client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount, opts.RetryCount);
         }
         catch { /* best-effort; older OSes may not expose the granular knobs */ }
     }
 
+    /// <summary>
+    ///     Driver.Modbus-009: cast a <see cref="TimeSpan"/> to a whole number of seconds with a
+    ///     minimum of 1 — protects callers from the int-cast truncation that turned 500&#160;ms
+    ///     keep-alive timing into "use the default" on most OSes.
+    /// </summary>
+    internal static int ClampToWholeSeconds(TimeSpan ts)
+    {
+        var seconds = (int)Math.Ceiling(ts.TotalSeconds);
+        return seconds < 1 ? 1 : seconds;
+    }
+
     public async Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
     {
         if (_disposed) throw new ObjectDisposedException(nameof(ModbusTcpTransport));