Fix structured message decoder to properly handle partial headers across chunks

- Rewrote StructuredMessageDecoder with proper pending buffer management
- Added decodeChunk() API that returns DecodeResult with status codes
- Never read partial headers - wait for complete bytes
- Track lastCompleteSegmentStart correctly for smart retry
- Simplified StorageContentValidationDecoderPolicy to use new decoder API
- Added comprehensive unit tests for split header scenarios

Co-authored-by: gunjansingh-msft <[email protected]>

Author: Copilot