Improve fallback agent system prompt and response formatting

[mingshl]

Originally created by @mingshl in mingshl/opensearch-agent-server#21

Description

The fallback agent (src/agents/fallback_agent.py) has a minimal system prompt. It needs refinement for better user experience.

Current state

• Basic system prompt (~5 lines)
• No response formatting guidelines
• No tool usage guardrails

Improvements needed

• Detailed system prompt with persona, capabilities, and limitations
• Response formatting guidelines (markdown, code blocks, tables for data)
• Tool usage guardrails (when to use which MCP tools)
• Error message formatting (user-friendly, not raw exceptions)
• Context-awareness (leverage page context for better responses)

References

• Source: src/agents/fallback_agent.py