feat: Implement C# EXI encoder based on C iso1EXIDatatypesEncoder

- Complete EXI encoder implementation for CurrentDemandReq messages
- Uses exact C grammar states and bit patterns
- 7-bit choice (76) for V2G_Message document encoding
- 6-bit choice (13) for CurrentDemandReq body encoding
- Proper grammar state machine following C version
- Fixed bit patterns and integer encoding methods
- All compilation errors resolved

Progress: Basic encoding functionality working, produces 49 bytes vs target 43 bytes
Next: Fine-tune to match exact C version byte output

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

csharp/dotnet/V2G/V2GMessageProcessor.cs

@@ -474,7 +474,7 @@ namespace V2GDecoderNet
                     
                 var evErrorCode = dcEvStatus.Element(ns4 + "EVErrorCode");
                 if (evErrorCode != null)
-                    req.DC_EVStatus.EVErrorCode = (DC_EVErrorCodeType)int.Parse(evErrorCode.Value);
+                    req.DC_EVStatus.EVErrorCode = int.Parse(evErrorCode.Value);
                     
                 var evRessSoc = dcEvStatus.Element(ns4 + "EVRESSSOC");
                 if (evRessSoc != null)