feat: Add ADK Visual Agent Builder with callbacks support

[wyf7107]

This PR introduces a comprehensive visual builder for ADK agents with the following features:

Core Features

• Visual workflow designer for agent creation
• Support for multiple agent types (LLM, Sequential, Parallel, Loop, Workflow)
• Agent tool support with nested agent tools
• Built-in and custom tool integration
• Callback management for all ADK callback types (before/after agent, model, tool)
• Assistant to help you build your agents with natural language
• Assistant proposes and writes agent configuration yaml files for you
• Save to test with chat interfaces as normal
• Build and debug at the same time in adk web!

UI/UX Improvements

• Builder mode with canvas-based visual editing using ngx-vflow
• Side panel with builder tabs for agent configuration
• Tooltips and improved button styling
• Builder assistant panel with AI-powered suggestions
• Breadcrumb navigation for agent hierarchy
• Responsive layout with draggable and zoomable canvas

Bug Fixes

• Fix builder icon disable logic when agent not built by builder
• Fix mode parameter clearing when switching apps to prevent auto-entering builder mode
• Add null safety checks to video service
• Fix callback YAML generation to use plural form (before_agent_callbacks)
• Fix isLoadingApps signal access in side-panel
• Fix various change detection and reactivity issues

Testing

• Add comprehensive test coverage for builder components
• Add missing providers (AgentBuilderService, FeatureFlagService, MatSnackBar)
• Fix all failing tests with proper mocks
• Add callback management tests