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/ModbusDriverFactoryExtensions.cs

@@ -111,16 +111,22 @@ public static class ModbusDriverFactoryExtensions
         var name = t.Name ?? throw new InvalidOperationException(
             $"Modbus config for '{driverInstanceId}' has a tag missing Name");
 
+        // Driver.Modbus-009: a String tag with StringLength = 0 yields RegisterCount = 0, which
+        // turns into an FC03/FC04 with quantity 0 — a spec-illegal request the PLC rejects with
+        // exception 03. Catch the misconfiguration at bind time with a clear diagnostic instead
+        // of waiting for the cryptic Illegal Data Value to surface at runtime.
+
         // AddressString takes precedence over the structured fields (Region/Address/DataType/
         // ByteOrder/BitIndex/StringLength/ArrayCount). Tags can mix forms freely — newer pasted
         // rows use the grammar string, legacy rows keep the structured form. Fields not derivable
         // from the grammar (Writable, WriteIdempotent, StringByteOrder) always come from the DTO.
+        ModbusTagDefinition tag;
         if (!string.IsNullOrWhiteSpace(t.AddressString))
         {
             if (!ModbusAddressParser.TryParse(t.AddressString, family, melsecSubFamily, out var parsed, out var parseError))
                 throw new InvalidOperationException(
                     $"Modbus tag '{name}' in '{driverInstanceId}' has invalid AddressString '{t.AddressString}': {parseError}");
-            return new ModbusTagDefinition(
+            tag = new ModbusTagDefinition(
                 Name: name,
                 Region: parsed!.Region,
                 Address: parsed.Offset,
@@ -137,26 +143,45 @@ public static class ModbusDriverFactoryExtensions
                 Deadband: t.Deadband,
                 UnitId: t.UnitId);
         }
+        else
+        {
+            tag = new ModbusTagDefinition(
+                Name: name,
+                Region: ParseEnum<ModbusRegion>(t.Region, t.Name, driverInstanceId, "Region"),
+                Address: t.Address ?? throw new InvalidOperationException(
+                    $"Modbus tag '{t.Name}' in '{driverInstanceId}' missing Address"),
+                DataType: ParseEnum<ModbusDataType>(t.DataType, t.Name, driverInstanceId, "DataType"),
+                Writable: t.Writable ?? true,
+                ByteOrder: t.ByteOrder is null
+                    ? ModbusByteOrder.BigEndian
+                    : ParseEnum<ModbusByteOrder>(t.ByteOrder, t.Name, driverInstanceId, "ByteOrder"),
+                BitIndex: t.BitIndex ?? 0,
+                StringLength: t.StringLength ?? 0,
+                StringByteOrder: t.StringByteOrder is null
+                    ? ModbusStringByteOrder.HighByteFirst
+                    : ParseEnum<ModbusStringByteOrder>(t.StringByteOrder, t.Name, driverInstanceId, "StringByteOrder"),
+                WriteIdempotent: t.WriteIdempotent ?? false,
+                ArrayCount: t.ArrayCount,
+                Deadband: t.Deadband,
+                UnitId: t.UnitId);
+        }
 
-        return new ModbusTagDefinition(
-            Name: name,
-            Region: ParseEnum<ModbusRegion>(t.Region, t.Name, driverInstanceId, "Region"),
-            Address: t.Address ?? throw new InvalidOperationException(
-                $"Modbus tag '{t.Name}' in '{driverInstanceId}' missing Address"),
-            DataType: ParseEnum<ModbusDataType>(t.DataType, t.Name, driverInstanceId, "DataType"),
-            Writable: t.Writable ?? true,
-            ByteOrder: t.ByteOrder is null
-                ? ModbusByteOrder.BigEndian
-                : ParseEnum<ModbusByteOrder>(t.ByteOrder, t.Name, driverInstanceId, "ByteOrder"),
-            BitIndex: t.BitIndex ?? 0,
-            StringLength: t.StringLength ?? 0,
-            StringByteOrder: t.StringByteOrder is null
-                ? ModbusStringByteOrder.HighByteFirst
-                : ParseEnum<ModbusStringByteOrder>(t.StringByteOrder, t.Name, driverInstanceId, "StringByteOrder"),
-            WriteIdempotent: t.WriteIdempotent ?? false,
-            ArrayCount: t.ArrayCount,
-            Deadband: t.Deadband,
-            UnitId: t.UnitId);
+        ValidateStringLength(tag, driverInstanceId);
+        return tag;
+    }
+
+    /// <summary>
+    ///     Driver.Modbus-009: reject <c>StringLength = 0</c> for <c>String</c>-typed tags. The
+    ///     driver computes <c>RegisterCount = (StringLength + 1) / 2</c> which would emit an
+    ///     FC03/FC04 with <c>quantity = 0</c>, a spec-illegal request the PLC rejects with
+    ///     exception 03 (Illegal Data Value). Surface as a clear bind-time error.
+    /// </summary>
+    private static void ValidateStringLength(ModbusTagDefinition tag, string driverInstanceId)
+    {
+        if (tag.DataType == ModbusDataType.String && tag.StringLength < 1)
+            throw new InvalidOperationException(
+                $"Modbus tag '{tag.Name}' in '{driverInstanceId}' has DataType=String but StringLength={tag.StringLength}. " +
+                $"String tags must declare StringLength >= 1 (the number of ASCII characters, packed 2 per register).");
     }
 
     private static T ParseEnum<T>(string? raw, string? tagName, string driverInstanceId, string field) where T : struct, Enum