feat: Implement ProcessWorker for message consumption

[artcava]

Description

Implements the background worker for consuming and processing messages from RabbitMQ queue, completing the asynchronous process execution pipeline.

Changes

Core Domain

• ProcessContext: New domain model for handler execution context with metadata support and cancellation token
• ProcessMessage.Metadata: Added metadata property for message routing and tracking

Server Components

• ProcessWorker: Background service implementing BackgroundService for long-running message consumption
  • Integrates with IMessageConsumer using callback pattern
  • Full ACK/NACK strategy with MessageContext.AcknowledgeAsync() and RejectAsync()
  • Comprehensive error handling with retriable vs non-retriable distinction
  • Graceful shutdown support
  • Separate handling for deserialization errors (JsonException)

Testing

• ProcessWorkerTests: Unit tests covering constructor validation and basic lifecycle

Configuration

• Program.cs: Registered ProcessWorker as hosted service

Architecture Notes

• Uses IProcessService facade instead of direct repository access for better separation of concerns
• Leverages existing MessageContext with delegate-based ACK/NACK
• Compatible with existing RabbitMqConsumer implementation
• ProcessContext created but not yet integrated (reserved for future handler enhancement)

Message Flow

RabbitMQ → ProcessMessage → Deserialize → TransitionToProcessing
    ↓
GetHandler → ExecuteAsync → CompleteProcess → ACK
    ↓
Error (retriable) → FailProcess → NACK(requeue=true)
Error (non-retriable) → FailProcess → NACK(requeue=false)
JsonException → NACK(requeue=false) → DLQ

Testing

• ✅ Unit tests pass
• ✅ Project compiles successfully
• ⏳ Integration tests pending (requires RabbitMQ instance)

Related Issues

Closes #99

Checklist

• Code follows project coding conventions
• Self-review completed
• Comments added for complex logic
• Unit tests added/updated
• All tests passing locally
• No new warnings introduced
• Documentation updated (if needed)
• Integration tests performed (pending infrastructure)

Breaking Changes

None

Migration Notes

No migration required. The worker is automatically registered and will start consuming messages when the Server application starts.