Expose artifacts in Agentflow variable resolution

[charliesheh]

Summary
Updated Agentflow variable resolution to expose node artifacts in addition to content and HTTP response data.

Changes Made:

• Added nodeOutput?.artifacts fallback in resolveVariables
• Allows downstream variable references to access artifacts when content and HTTP data are unavailable

Testing

• Verified lint passes successfully
• Confirmed minimal diff limited to buildAgentflow.ts

[gemini-code-assist]

Code Review

This pull request updates the resolveVariables function in packages/server/src/utils/buildAgentflow.ts to include nodeOutput?.artifacts as a fallback when determining the actual value of a node output. This ensures that artifact data is correctly captured during variable resolution. I have no feedback to provide as there are no review comments.