fix: collect eval state from workflow nodes

[he-yufeng]

Summary

Fixes #5995.

create_empty_state() only walked sub_agents, so graph-based Workflow roots crashed when the dev server tried to add the current session to an eval set. Workflow children live in workflow.graph.nodes, not sub_agents.

This updates the state traversal to:

• keep the existing sub_agents walk for normal agents
• also walk graph.nodes for Workflow-style roots and nested graph nodes
• track visited objects so shared graph/agent nodes are not processed repeatedly

Testing

python -m pytest tests\unittests\cli\utils\test_state.py -q
python -m py_compile src\google\adk\cli\utils\state.py tests\unittests\cli\utils\test_state.py
python -m pyink --check src\google\adk\cli\utils\state.py tests\unittests\cli\utils\test_state.py
git diff --check

[adk-bot]

I have successfully performed a comprehensive, read-only analysis of Pull Request #6001 and its related issue #5995.

I have created a premium, structured review report for you. You can read the full, detailed analysis in the newly generated artifact:
👉 pr_analysis_report.md

Key Highlights & Summary of Findings

1. Google CLA Compliance Gate (Passed):
   • Verified that the contributor @he-yufeng has successfully signed and passed the standard Google CLA check (re-verified via standard GitHub checks status).
2. Root Cause Resolved:
   • The state initialization utility create_empty_state() in state.py previously assumed the root agent is a standard BaseAgent and traversed child elements via .sub_agents.
   • In graph-based Workflows (introduced in ADK 2.x), child nodes live in .graph.nodes, which produced an AttributeError and crashed the ADK Dev Server with a 500 Internal Server Error when developers attempted to save active sessions to evaluation sets.
   • The PR resolves this systematically by dynamically querying and traversing both sub_agents and graph.nodes. It also elegantly handles cyclic or shared node graphs using an object identity cycle-tracker (visited set).
3. Principles & Style Compliance (Pass with Nits):
   • Quality: The solution is robust and does not introduce any breaking changes to the public API namespace.
   • Testing: Excellent behavior-focused test coverage added in the new unit test file, utilizing 100% real components (Workflow, LlmAgent, and edge bounds) without mocking.
   • Styling/Typing Nits:
     • Nit 1: The new test file tests/unittests/cli/utils/test_state.py is missing the mandatory from __future__ import annotations header import.
     • Nit 2: Parameter typing in create_empty_state(...) was relaxed to Any instead of leveraging ADK's BaseNode interface (the common ancestor for both BaseAgent and Workflow), which could be annotated more strongly.

Next Steps and Recommendations

• Recommendation: Approve with Nits. The patch is high-impact, low-risk, and handles DAG structures extremely cleanly. We can request