Implement NodeBehavior trait system for extensible node types #100

Overview

Create the NodeBehavior trait system that enables extensible, type-specific logic for different node types while maintaining runtime polymorphism.

Implementation Tasks

• Define NodeBehavior trait with core methods
• Create NodeBehaviorRegistry for managing behavior instances
• Add trait methods for validation, content processing, and node capabilities
• Implement registration and lookup mechanisms
• Add error handling for missing behaviors
• Create extensibility patterns for plugins

Acceptance Criteria

• NodeBehavior trait defines clear contract for node type logic
• Registry supports registration and retrieval of behaviors
• Thread-safe implementation for concurrent access
• Validation method prevents invalid node states
• Content processing allows type-specific transformations
• Capability queries (can_have_children, supports_markdown, etc.)
• Error handling for unknown node types
• Plugin-ready architecture for external node types

Technical Specifications

NodeBehavior Trait:

pub trait NodeBehavior: Send + Sync {
    fn type_name(&self) -> &'static str;
    fn validate(&self, node: &Node) -> Result<(), ValidationError>;
    fn can_have_children(&self) -> bool;
    fn supports_markdown(&self) -> bool;
    fn process_content(&self, content: &str) -> Result<String, ProcessingError>;
    fn default_metadata(&self) -> serde_json::Value;
}

Registry Implementation:

pub struct NodeBehaviorRegistry {
    behaviors: HashMap<String, Box<dyn NodeBehavior + Send + Sync>>,
}

Dependencies:

• std::collections::HashMap
• async-trait for async behavior methods
• thiserror for custom error types
• Send + Sync bounds for thread safety

This enables the polymorphic node system that supports both built-in and user-defined node types.