feat: Add mock servers for DGX H100, H200, and B200 systems

This commit adds comprehensive mock implementations for three additional
NVIDIA GPU architectures to support multi-architecture testing:

- dgxh100: 8-GPU DGX H100 system with Hopper architecture
- dgxh200: 8-GPU DGX H200 system with Hopper architecture
- dgxb200: 8-GPU DGX B200 system with Blackwell architecture

Each mock server provides:
- Realistic device properties (memory, compute capability, architecture)
- Complete MIG profile support with architecture-specific configurations
- Full NVML interface implementation following existing dgxa100 patterns
- Thread-safe operations with proper locking
- Comprehensive MIG placement and profile information

Key Features:
- H100/H200: 141GB memory, Hopper architecture, enhanced MIG profiles
- B200: 192GB memory, Blackwell architecture, current-gen capabilities
- Consistent API with existing dgxa100 mock server
- Proper error handling and return codes
- Realistic PCI bus IDs and device UUIDs

This enables comprehensive testing across multiple GPU architectures
without requiring physical hardware.

Signed-off-by: Fabien Dupont <[email protected]>

Author: fabiendupont