fix: Duplicate tool calls showing in assistant UI

[BinaryMuse]

Changes to the history events and system prompts for the AI system causes duplicate tool calls to show up in the UI. This PR fixes that, as well as the system prompt showing up in the UI.

[greptile-apps]

Greptile Overview

Greptile Summary

Fixes duplicate tool calls and system messages appearing in the AI assistant UI by filtering messages on display and checking for existing tool call IDs before adding new ones.

• Introduced visibleMessages filter in AIAssistant.tsx to hide system messages from UI
• Added deduplication logic in useAIChat.ts to prevent duplicate tool call messages during history restoration
• Replaced all messages.length checks with visibleMessages.length for UI logic

Important Files Changed

Filename	Overview
src/components/runbooks/editor/ui/AIAssistant.tsx	Filters system messages from UI display using useMemo, prevents system prompts from showing to users
src/lib/ai/useAIChat.ts	Prevents duplicate tool call messages by checking existing IDs before adding to message list

[greptile-apps]

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