Implement GraphQL Subscriptions Support

[konard]

Summary

This PR implements comprehensive GraphQL subscription support for real-time notifications of link changes in the Data.Doublets.Gql project.

Features Implemented

• Real-time Subscriptions: Four subscription types for different use cases:

  • link_created: Notifies when new links are created
  • link_updated: Notifies when existing links are modified
  • link_deleted: Notifies when links are deleted (returns deleted link ID)
  • link_changed: Combined notifications for both created and updated links

• Filtering Support: All subscriptions support where clause filtering using the existing LinksBooleanExpression input type

  • Filter by id, from_id, to_id
  • Additional complex filtering through existing boolean expression structure

• Reactive Streams: Uses System.Reactive for proper observable stream handling

• Event Integration: Mutation resolvers automatically trigger subscription notifications

• WebSocket Transport: Leverages existing WebSocket infrastructure for real-time communication

Technical Implementation

• Updated LinksSubscription class with reactive stream resolvers using IObservable<T>
• Modified LinksMutation to emit events to subscription streams on create/update/delete operations
• Added System.Reactive package dependency for proper reactive extensions support
• Maintained full backward compatibility - no breaking changes to existing APIs

Example Usage

# Subscribe to new link creation
subscription {
  link_created(where: { from_id: { _eq: 1 } }) {
    id
    from_id  
    to_id
    type_id
  }
}

# Subscribe to all link changes
subscription {
  link_changed {
    id
    from_id
    to_id
    type_id
  }
}

Testing

• All existing tests pass (8/8)
• Build completes successfully with no compilation errors
• Server configuration already supports WebSocket subscriptions through existing packages
• Added comprehensive test documentation in examples/subscription_test.md

🤖 Generated with Claude Code

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Resolves #31