Add non-streaming fallback and unified plan-result handling for workflow planning

[zhongkaifu]

Motivation

• The front-end could fail to start building a workflow when the streaming planning endpoint is unavailable or errors; the UI lacked a consistent fallback and unified handling of plan results.
• Ensure the UI always updates canvas/editor/status and surfaces logs/suggestions even when SSE streaming fails.
• Improve resilience by falling back to the non-streaming POST /api/plan endpoint.

Description

• Implemented a shared result handler applyPlanOutcome to centralize UI updates (update editor, canvas, status, run button state and chat messages).
• Added requestPlanFallback which calls the non-streaming POST /api/plan endpoint, renders returned logs, and forwards results into applyPlanOutcome.
• Updated requestPlan to keep using the streaming endpoint but catch stream errors and automatically call the fallback path; streaming and fallback now produce consistent UI state and messages.
• All changes are in webapp/js/network.js (new functions: applyPlanOutcome, requestPlanFallback; refactor: requestPlan).

Testing

• Automated tests: none were run for this change.
• (No CI/test output was produced during this rollout.)

---

Codex Task

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, you can upgrade your account or add credits to your account and enable them for code reviews in your settings.