feat: stream intermediate agent reasoning and token-by-token final answers

[knoxiboy]

📋 PR Checklist

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🔗 Related Issue

Closes #564

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📝 What does this PR do?

Streams intermediate ReAct agent reasoning steps (thoughts, tool actions, and observations) in the WebSocket/SSE stream, and progressive token-by-token final answers in the chat stream.

Also updates the React frontend (ChatPanel.tsx and MessageBubble.tsx) to handle, parse, and display the intermediate reasoning steps within a collapsible animated accordion component.

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🗂️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

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🧪 How was this tested?

• Ran the backend locally
• Ran the frontend locally
• Tested the affected API endpoints manually

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📸 Screenshots (if UI change)

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⚠️ Anything to flag for reviewers?

None.

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed