Auto: s7-a2 — STRING/WSTRING/CHAR/WCHAR

Closes the NotSupportedException cliff for S7 string-shaped types.

- S7DataType gains WString, Char, WChar members alongside the existing
  String entry.
- New S7StringCodec encodes/decodes the four wire formats:
    STRING  : 2-byte header (max-len + actual-len bytes) + N ASCII bytes
              -> total 2 + max_len.
    WSTRING : 4-byte header (max-len + actual-len UInt16 BE) + N×2
              UTF-16BE bytes -> total 4 + 2 × max_len.
    CHAR    : 1 ASCII byte (rejects non-ASCII on encode).
    WCHAR   : 2 UTF-16BE bytes.
  Header-bug clamp: actualLen > maxLen is silently clamped on read so
  firmware quirks don't walk past the wire buffer; rejected on write
  to avoid silent truncation.
- S7Driver.ReadOneAsync / WriteOneAsync issue ReadBytesAsync /
  WriteBytesAsync against the parsed Area / DbNumber / ByteOffset and
  honour S7TagDefinition.StringLength (default 254 = S7 STRING max).
- MapDataType returns DriverDataType.String for the three new enum
  members so OPC UA discovery surfaces them as scalar strings.

Tests: 21 new cases on S7StringCodec covering golden-byte vectors,
encode/decode round-trips, the firmware-bug header-clamp, ASCII-only
guard on CHAR, and the StringLength default. 85/85 passing.

Closes #288

src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7DriverOptions.cs

@@ -116,5 +116,11 @@ public enum S7DataType
     Float32,
     Float64,
     String,
+    /// <summary>S7 WSTRING: 4-byte header (max-len + actual-len, both UInt16 big-endian) followed by N×2 UTF-16BE bytes; total wire length = 4 + 2 × StringLength.</summary>
+    WString,
+    /// <summary>S7 CHAR: single ASCII byte.</summary>
+    Char,
+    /// <summary>S7 WCHAR: two bytes UTF-16 big-endian.</summary>
+    WChar,
     DateTime,
 }