Detect isolated nodes in finalize_workflow and prompt remediation

[zhongkaifu]

Motivation

• Ensure workflows submitted via finalize_workflow are not leaving behind truly isolated nodes (zero indegree and zero outdegree) that likely indicate missing bindings.
• Prompt the planner/LLM to analyze and fix nodes that should reference other node fields rather than silently accepting orphaned nodes.
• Provide actionable feedback so authors can use update tools to add params, depends_on, or branch targets to form connections.
• Improve gating so finalization requires explicit resolution or justification of isolated nodes.

Description

• Add a new helper _find_isolated_nodes(workflow) which computes indegree/outdegree using infer_edges_from_bindings and returns nodes with zero in/out degree.
• Add _build_isolated_nodes_feedback_message to generate human/LLM-readable guidance about isolated nodes and suggested remediation tools like update_action_node, update_condition_node, and update_loop_node.
• Update the planner system prompt to mention that finalize_workflow will check for isolated nodes and instruct authors to fix them before finalizing.
• Modify finalize_workflow to call the new detection, include isolated_nodes in the returned payload, append isolation feedback to feedback, and mark should_continue when isolation review is required.

Testing

• No automated tests were executed for this change.
• Static checks and type expectations were not run as part of this PR.
• Manual lint/formatting checks were not run as part of this PR.
• Further CI/test runs should be added to validate detection behavior and integration with LLM-driven remediation tools.

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Codex Task