IAV-91/feat: Add parent agent relationship to Agent entity

Summary

This MR adds support for hierarchical agent structures by introducing a parent-child relationship within the Agent entity.

Changes

• Added parentAgent field with @ManyToOne relationship to the Agent entity
• Configured lazy fetching to optimize query performance
• Enables agents to reference a parent agent for inheritance or delegation patterns

Technical Details

• Self-referential relationship using @JoinColumn(name = "parent_agent_id")
• Foreign key stored in parent_agent_id column
• Fetch type set to LAZY to prevent unnecessary data loading

Database Impact

• New column: parent_agent_id (nullable) in agents table
• Foreign key constraint to agents.id****

Closes IAV-91