Restore update workflow planner test and remove references to start node

[zhongkaifu]

Motivation

• Reinstate the removed planner test while eliminating hardcoded references to a start node to match updated workflow conventions.
• Ensure the planner update flow is still exercised against an existing graph shape rather than a special start node.

Description

• Re-added tests/test_update_workflow_planner.py and replaced the original start node with a seed node in the test fixture.
• Updated the fake planner to produce a summarize node that depends on seed and that uses {{ result_of.seed.message }} for its params.
• Adjusted the action_registry and all assertions to reference demo.seed and seed instead of demo.start and start.
• Kept the same test scaffolding that monkeypatches plan_workflow_structure_with_llm, analyze_user_requirement, _ensure_actions_registered_or_repair, and _validate_and_repair_workflow to isolate planner behavior.

Testing

• No automated tests were executed for this change.

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Codex Task