fix: prevent page crash when creating invalid node connections in agent editor

[skyvast404]

Summary

• Add defensive validation in onConnect handler: check source/target nodes exist, prevent self-loops, and wrap the entire handler in try/catch to gracefully handle unexpected errors
• Wrap BuilderCanvas with the project's existing ErrorBoundary component so that graph rendering errors are isolated to the canvas area instead of crashing the entire page

Closes #58

Root Cause

The onConnect handler in builderStore.ts had no validation or error handling. When users created invalid connections between nodes (e.g. connecting incompatible node types), errors thrown by determineEdgeTypeAndRouteKey or addEdge would propagate uncaught, causing a React render crash that took down the full page. There was also no ErrorBoundary around the BuilderCanvas component.

Test plan

• Create valid connections between nodes — should work as before
• Try creating invalid or unusual connections — page should not crash
• If a rendering error occurs in the canvas, ErrorBoundary should show fallback UI with "Try again" button
• Toolbar and sidebar should remain functional even if canvas encounters an error

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